docs: should not need index files in every parent sidebar element

[jorgeorpinel]

From #727 (comment) with e.g. /doc/user-guide/contributing
Also related to #305

Using "source": false, on sidebar.json doesn't work unless it is the top level (e.g. get-started or understanding-dvc). In other levels, this breaks the engine.

Consider also automatically generating a list of H2s in child documents when no source field is used for a parent slug.

• And finish docs: remove unnecessary index pages that serve as a table of contents #727 after this.