Currently, in addition to building (Markdown) pages via main.html, we have hardcoded provisions for theme templates to build 404.html, search.html, and sitemap.xml. However, there is no way for a theme to provide any additional pages (sitemap.html or Sphinx's genindex.html).

We do have the extra_templates config setting, but that specifically assumes the template files are in the docs_dir, not at a path in the Jinja environment (they go through mkdocs.commands.build.build_extra_templates() rather than mkdocs.commands.build.build_template()).

In the work on Plugins (#206), a plugin can alter the config and add an additional directory to the Jinja Environment. That same plugin can also add a hook which explicitly calls mkdocs.commands.build.build_template() with a template in its template dir (that's how I implemented search as a plugin). However, themes have no way to alter the config.

To allow something like this, we need some way to allow configuration for themes. I see a few possibilities:

1. The theme could include a config file which defines specific variables. For example an extra_templates variable could include a list of additional templates to pass through build_template. As prior art, Sphinx themes provide for a config.ini file in their root dir. We might want to use a .yml file instead to match our config file format. Or we could use Python in the required __init__.py file.

2. We could allow nested config settings in mkdocs.yml. Following existing conventions, perhaps something like:

   theme:
       themename:
           - extra_templates: [sitemap.html]
           - include_sidebar: false

3. A combination of the two. The theme author could define some defaults and the user could override them. This would also make a great way for themes to define optional behavior (include_sidebar which defaults to True but the user can set to False).

Yes, we have the extra config setting. While that is already used by third party themes to enable/disable theme features, the theme has to provide for the setting not being defined at all. And extra does not provide a way for a theme to pass more templates through build_template.

If we just want to allow a theme to pass more pages through build_template, then option 1 makes the most sense to me. But personally, I would prefer option 3. Option 2 would work, but the user would need to explicitly set each setting before it would be defined at all.