Multiple features

[danirus]

This PR brings many fixes and features:

• Add breadcrumbs.
• Use smooth scrolling.
• Add back-to-top button.
• Add customizable header links.
• Limit height of version dropdown.
• Use compact notation to display number of stars, and forks, in header's repository widget.
• Scroll the left sidebar to show the link corresponding to the current page.
• In the sphinx-nefertiti documentation website, reset the colorset selected by the user 24 hours after it has been selected.
• Migrate from setup.py and multiple requirements files, to pyproject.toml.
• Use tox to run verify that tests pass against multiple versions of Sphinx.