Write content guidelines

[NathanLovato]

Quoting from #3390

The quality and the style of each page varies a lot as a result. Some pages are long and hard to read, others are incomplete or unclear. Many pages don't follow our writing guidelines.

That is in part because we do not have content guidelines. As reviewers, we often don't know what we should merge or not.

The content guidelines would be a short document, a starting point for all contributors that defines:

1. The type of content we do and don't want in the documentation.
2. The recommended length for pages.

The page should link to other guidelines, like the code style guides and writing style guides.