Development Documentation Restructuring

The API docs for the pybuild module have been moved into the main documentation to make them easier to find.

I've consolidated the development documentation into a single directory, and recreated "Contributing to Portmod" as a sort of intro page.

• I sort of want to have CONTRIBUTING.md just link to that, but it will probably need to remove the sphinx references in favour of regular urls. Doesn't really work. It will just be a page with a link to the docs instead.
• Some stuff still needs to be filled out.
• Use flag aliases should be added to the use flag guide.
• Use flag defaults need to be documented