Add documentation linter tooling

I read that the documentation tries to follow the Microsoft writing style guide. https://docs.funkwhale.audio/develop/contributor_documentation/documentation.html#style-guide

I think we can provide some help to users writing documentation by adding some tooling, such as a linter.

I have been using https://github.com/errata-ai/vale for linting documentation, the tool is really powerful, and is able to mix multiple styling guide or you can also provide your own rules (e.g. how to properly write Funkwhale).

Mixing styles is not really the goal, as they may be incompatible, but it shows the flexibility of the tool.