feat: add the indices to the optional sidebar components

[12rambau]

Fix #316

It has been requested a long time ago but better late than sorry ;-)

So I created a small indices.html and the according _indices.scss. It is build to be added in the left_sidebar_end:

"left_sidebar_end": ["indices.html", "sidebar-ethical-ads.html"]

I decided not to add it in our default documentation, maybe we should ? In the meantime here are 2 screenshot from one of my documentation:

[choldgraf]

This looks good to me - my one suggestion is that we document this so that it's a bit more discoverable. Maybe we could add a section to Navigation and links to note how to add indices to the sidebar?

[12rambau]

@choldgraf I added the requested extra page. There is now a warning in the build (I think it's unrelated to my modification) but I didn't find where the transform member is called. Do you have an idea ?