Describe doc-check in developer manual

Move existing documentation for the doc-check target into the developer
manual, and extend it to cover also the include sorter.  Add a separate
page that explains the include order enforced by the checker.
Various updates to make the description match better the current state
of affairs, with the module dependency graph embedded into the Doxygen
documentation.

Change-Id: I570e4f6af78453cffea89855f294a599f28dedd9