Update sphinx theme

[danirus]

This PR is about updating the theme used for the documentation:

• Update to sphinx-nefertiti 0.6.0
• Update docs/conf.py with example header links.
• At the time prompt_toolkit was in version 3.0.43, ReadTheDocs stopped sharing the list of versions available for packages in the HTML context of the Sphinx theme. In this PR the list of versions is given in the conf.py file.
• Fix some warnings displayed by Sphinx when building the documentation.

[jonathanslenders]

Thank you!