Rename and add deprecation warnings for section names

[12rambau]

rebase from #789 initialy proposed by @choldgraf:

This adds some renaming in the docs and checks for old configuration values since we are standardizing our naming of the primary/secondary sidebars.

[choldgraf]

Thanks so much for picking this up 🙏

It looks good to me. IMO we should merge it relatively quickly because it's going to go out of date quickly as well