I'm currently working on a C# project and VisualAssist generates these fancy /// <summary></summary>
comments for me and I've been using them do document my code.
I assume there must be a way to use these comments to create HTML documentation like those done by Doxygen or Javadoc. How do I do that?
This page might interest you: http://msdn.microsoft.com/en-us/magazine/dd722812.aspx
Doxygen or Sandcastle help file builder are the primary tools that will extract XML documentation into HTML (and other forms) of external documentation.
Note that you can combine these documentation exporters with documentation generators - as you've discovered, Resharper has some rudimentary helpers, but there are also much more advanced tools to do this specific task, such as GhostDoc (for C#/VB code with XML documentation) or my addin Atomineer Pro Documentation (for C#, C++/CLI, C++, C, VB, Java, JavaScript, TypeScript, JScript, PHP, Unrealscript code containing XML, Doxygen, JavaDoc or Qt documentation).
In 2017, the thing closest to Javadoc would probably DocFx which was developed by Microsoft and comes as a Commmand-Line-Tool as well as a VS2017 plugin.
It's still a little rough around the edges but it looks promising.
Another alternative would be Wyam which has a documentation recipe suitable for net aplications. Look at the cake documentation for an example.
The above method for Visual Studio didn't seem to apply to Visual Studio 2013, but I was able to find the described checkbox using the Project Menu and selecting my project (probably the last item on the submenu) to get to the dialog with the checkbox (on the Build tab).