Add document structure marker for namespaces
Same as #167 but for namespaces.
Adding a document structure override will replace the "Additional documentation" blocks with a new index.
For instance:
/**
* GLib:
*
* ## Topics
*
* ### Overview
*
* - [Building GLib](building.html): how to compile GLib itself
* - [Compiling GLib Applications](compiling.html): how to compile applications using GLib
* - [Cross-compiling the GLib package](cross-compilation.html): how to cross-compile code using GLib
* - [Running GLib Applications](running.html): How to run and debug applications using GLib
*
* ### Fundamentals
*
* - [Version information](versioning.html): variables and functions to check the version of GLib
*
* ...
*/