How to create an admin page to add/remove elements

I have a form in my website that's used to insert documents (has different privileges -Admin/User- ).

This form includes a drop-down list, the problem is that it needs to be edited by the admin to make more or less items in the drop down list.

This edit shouldn't be a code edit (which is absolutely easy) but a form that allows the admin to add elements to drop-down list I searched around and I didn't find an answer , I wish you could help me with this !

<?php
require_once("identification.php");
require_once('connexionDB.php');
$nom            = isset($_POST['nom']) ? $_POST['nom'] : "";
$pole           = isset($_POST['pole']) ? $_POST['pole'] : "";
$valideur       = isset($_POST['valideur']) ? $_POST['valideur'] : "";
$perimetre      = isset($_POST['perimetre']) ? $_POST['perimetre'] : "";
$direction      = isset($_POST['direction']) ? $_POST['direction'] : "";
$activite       = isset($_POST['activite']) ? $_POST['activite'] : "";
$version        = isset($_POST['version']) ? $_POST['version'] : "";
$type_doc       = isset($_POST['type_doc']) ? $_POST['type_doc'] : "";
$description    = isset($_POST['description']) ? $_POST['description'] : "";
$zone           = isset($_POST['zone']) ? $_POST['zone'] : "";
$langue         = isset($_POST['langue']) ? $_POST['langue'] : "";
$date           = isset($_POST['date']) ? $_POST['date'] : "";
$comm_sur_modif = isset($_POST['comm_sur_modif']) ? $_POST['comm_sur_modif'] : "";
$commentaire    = isset($_POST['commentaire']) ? $_POST['commentaire'] : "";
$auteur         = $_SESSION["fati"];

if (isset($_FILES['document']) and !empty($_FILES['document']['name'])) {
    $taillemax        = 4221225472;
    $extensionvalides = ['pdf', 'docx'];
    if ($_FILES['document']['size'] <= $taillemax) {
        $extensionUpload = strtolower(substr(strrchr($_FILES['document']['name'], '.'), 1));
        if (in_array($extensionUpload, $extensionvalides)) {
            $chemain = "doc/" . $nom . "." . $extensionUpload;

            $resultat = move_uploaded_file($_FILES['document']['tmp_name'], $chemain);

            if ($resultat) {
                $requete  = "insert into document(nom,direction,pole,activite,version,type_doc,description,zone,perimetre,langue,chemin,auteur,date,comm_sur_modif,commentaire) values(?,?,?,?,?,?,?,?,?,?,?,?,?,?,?)";
                $params   = [
                    $nom,
                    $direction,
                    $pole,
                    $activite,
                    $version,
                    $type_doc,
                    $description,
                    $zone,
                    $perimetre,
                    $langue,
                    "doc/" . $nom . "." . $extensionUpload,
                    $auteur,
                    $date,
                    $comm_sur_modif,
                    $commentaire,
                ];
                $resultat = $pdo->prepare($requete);
                $resultat->execute($params);
                header("location:documents.php");
            }
        }
    }
}

<div class="form-group">
    <label for="type_doc">type de document  </label>
    </br>
    <select name="type_doc" id="type_doc">
        <option value="NA">N/A</option>
        <option  value="guide_de_conception">guide de conception</option>
        <option  value="standard_rt">standard et RT</option>
        <option  value="methodologies">methodologies</option>
        <option  value="processus">processus</option>
        <option  value="retex_capitalisation">retex et  capitalisation</option>
        <option  value="normes_reglementations">normes reglementations</option>
        <option  value="cdc">CDC</option>
        <option  value="essais_plans_validation">essais et plans de validation</option>
    </select>
</div>

Overview:

You need to store the list of desired fields somewhere. You cannot store them in LocalStorage or in cookies - those would be stored locally on the admin's computer (so how would a user see those changes?). You need a central location: the webserver. This gives you two choices: (a) a file on the webserver, or in a MySQL (now called MariaDB) database table. I suggest (b).
In order to read/write to a file on the webserver -- or to add/delete/read from a database on the webserver -- you need to write some server-side code. MOST webservers have PHP available as a back-end language, but Microsoft servers use ASP .Net. There is now also the choice to install/use node.js (if you want to use javascript as your back-end server language). As mentioned, PHP is hugely popular and there are somewhere around a gazillion blog and YouTube tutorials showing how to do this.
Using PHP as the example, you will rename your index.html to index.php - just do that. Nothing that currently exists will be affected - but now you can embed sections of PHP code and the server will run that code before rendering/displaying the HTML. (Note that nothing else changes when you rename the file extension. Provided you are on an apache web server - and most are - you need never use the .html extension ever again. Try it.)
Your index.php will now begin with a snippet of PHP code that tells it to (a) login to the database, (b) read that value from the table, (c) store the value in a variable. It is now patently easy to schlep that data into the HTML as the page is being rendered.
You will need a page that only the admin can access. Again, using a back-end language will allow your admin page (HTML) to request a username and password, and then run some back-end code to check the info stored on the webserver (again, either in a file or in a database table), to see if the username/password is correct.
Your admin page, after logging in, will:
- As in step (4) above, read the database table to get the current settings for user-level choices for the drop-down, then display those choices on the screen. You will also need a method of adding new options for the drop down, and a button to signify that the changes are done. Once pressed, the page will send that data back to the webserver to store back in the table, over-writing what was there before.
- There are two ways to send data from an HTML page to the webserver: (a) <form></form> and (b) AJAX. By all means, use AJAX - forms are more restricted, way less elegant, and require that the page is refreshed or changed. Forms are 1999, AJAX is 2019.
- AJAX, written in javascript/jQuery, lets you (a) detect a button click; (b) collect the data from the input fields; (c) send that data to a PHP file on the webserver; (d) receive a message (from the webserver, after it has finished adding the data to the table) back on the HTML side; (e) update the page smoothly and without refreshing anything. With AJAX, you (as the developer) retain complete control from beginning to end, comme il faut.