DOC: Remove related topics entries from the sidebar

[timhoffm]

PR Summary

This PR removes the "Related Topics" section from the sidebar:

This comes from the original sphinx theme on which the Matplotlib theme was based. I suppose nobody ever bothered to remove it, even though it does not add value for the Matplotlib docs and instead just increases clutter.

That section has two parts:

• the hierarchy to the current page. This is not needed because it's already available in the breadcrumbs at the top.
• the previous/next pages. Since our docs are not a continuous narrative, the previous and next pages are often actually not related.

[story645]

I think this may need to a be a dev call discussion but I'm also tentatively +1 on this change w/ option to have this sidebar where it is helpful.

[jklymak]

I agree that this is clutter particularly as we have bread crumbing at the top...