[docs] Put in-page header navigation in a secondary sidebar

[choldgraf]

Currently, the theme puts in-page header navigation in the primary sidebar to the left:

However, it has become standard across modern documentation engines to use a three-column layout and put the in-page navigation on the right. Some examples:

• Docasaurus
• Material for mkdocs
• GitBook
• MarkDoc

And within the Sphinx ecosystem:

• Furo
• Book theme
• PyData theme

I think that Sphinx's documentation would evoke a more modern theme UI if it similarly used a three-column layout, with the right-most column containing in-page navigation to headers.

[tk0miya]

+1