docs: change sphinx theme to Clarity

[liborjelinek]

I am proposing a new theme for documentation. This is a follow-up and closes #991. It includes the "Edit on GitHub" link and the version select dropdown.

The Clarity theme already has built-in support for version selection. It is less visually intrusive than today's banner

It is also JS-free because the version list is validated and prepared during the Sphinx build. It allowed me remove former JS and legacy jQuery.

I also explicitly list dependencies in requirements.txt (only Sphinx and Clarity theme).

Lastly, I have changed lsver.go script a bit to conform Clarity theme's expected structure (changed sort order (oldest→latest becomes latest→oldest) and changed output from {"entries": [...]} to an array of {"version": "..."} objects.

You can preview it at https://showcase.readcraft.io/syncthing/. But keep in mind that version selection doesn't work because, for faster working, I am deploying to the showcase website only the latest version's files.

[liborjelinek]

Because I introduced the first external dependency (the theme itself 😄), I had to change the HTML build step.

But we now have an egg-chicken problem here.

The theme requires a different structure for versions.json (changes to lsver.go are already in PR), but the updated JSON must be available at https://docs.syncthing.net/versions.json to successfully build HTML. Which is obviously not now, and thus this PR build is failing.