Since python doesn't have a way to lock deps for a particlar project
by default, I didn't bother with it. This resulted in mkdocs updating at
some point, bringing a breaking change to how tabs are declared and
broken tabs on docs-develop.pleroma.social. I've learned my lesson
and locked deps with pipenv in docs!5. This MR updates Pleroma
docs to use the new tab style, fortunately my editor did most of it.

Closes #2045

Not useful for source releases as we don't have a way to automate connecting to the running instance.

- [] for options
- <> for mandatory arguments
- foo ... when foo can be repeated

[ci skip]