nav: create title entry in right CONTENTS ToC sidebar

[jorgeorpinel]

Often there's a long intro in our docs before getting to the first H2 header (example below) but the CONTENTS navigation element in the right side bar does not list the document's H1 title, so it's not possible to scroll to the very top (in order to read the intro) from there, which I think would be a logical feature.

https://dvc.org/doc/tutorials/versioning

[jorgeorpinel]

I keep bumping into this issue. WDYT @shcheklein / @rogermparent, worth prioritizing soonish?

[rogermparent]

Seems simple enough, but we may want to consider a design other than simply adding the top heading to the content bar. There is an issue we must solve in how to handle pages that hide the content menu, i.e. pages with no headings besides the top h1.

For example, we could add a "Return to Top" button like in many other blogs; it could be located next to the "content" heading when the list is present, and maybe show standalone when on a page without subheadings.

[jorgeorpinel]

Note that I've been using a workaround in the docs repos (example). We should remove these if the feature gets implemented.