Add some initial minimal docs for each check

[swissspidy]

Each check should have a short description that can be displayed in the admin / CLI, and a link to a web page with a more detailed documentation, which can be more easily maintained.

See #448 See #316

To-do:

• [ ] Rephrase short descriptions
• [ ] Create new pages for some checks which don't have any documentation yet
• [ ] Maybe centralize documentation somewhere, e.g. under https://developer.wordpress.org/plugins/
• [ ] Actually display the documentation in the admin / CLI context