Improve the docs on `project.scala`

[Gedochao]

Is your feature request related to a problem? Please describe.
Extracted from #1489

Describe the solution you'd like

• the existing docs on project.settings.scala seem to have been confusing to some users, so probably improve the docs a bit
• we probably need a dedicated, small doc on this in the Guides section
• the Project root directory doc should link to the aforementioned guide
• mention project.scala in various spots in the docs to make it more visible