Resolve "Lots of small typos in documentation"

Related issue: #645 (closed)

This Merge Request includes:

• Tests
• A changelog fragment (cf https://docs.funkwhale.audio/contributing.html#changelog-management)

I assume some of these changes are due to the person writing it being French, but there's also a lot of changes that are obviously mistakes.

Ironically my commit message has a typo :P