Docs: Fix the way tabs are declared

Merged rinpatch requested to merge fix/docs-tabs into develop

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 I've learned my lesson and locked deps with pipenv in docs!5 (merged). This MR updates Pleroma docs to use the new tab style, fortunately my editor did most of it.

Closes #2045 (closed)

Merge request reports