In-code rST documentation moved to docs for consistent handling and cross-referencing

This MR changes nothing in code! Only documentation related tidy up. Completely save to merge at any point.

There was previously 2 stand-alone documents for developers written in rST inside the code source tree.

Instead of keeping it here and copying to docs on every build they are now moved to docs tree and links to the rendered version are added here. This allows to modify those docs with cross-referencing, etc, that otherwise will look disambiguate here.

The description in archery-manage help message (relevant for rendered docs) also updated.