GitLab

- ps-menurc removed in GIMP 2.7.2 (i.e. stable 2.8.0) according to
  NEWS.pre-2-8.
- in our main README, redirect now to devel-docs/README.md as entry
  point for contributors.
- move HACKING to devel-docs/ and specialize it into a "how to build
  from git and contributed" docs:
  * Make it markdown.
  * Remove some now obsolete or redundant recommendations (to be fair,
    we can probably clean up the file a bit more, but we'll see).
  * Add/improve relevant information.
  * Make more obvious when you want to build from tarball or git.
  * Just keep a single short paragraph about the coding style to
    redirect people to the appropriate CODING_STYLE.md file.
  * Don't recommend sending patches to the mailing list anymore.
- move CODING_STYLE.md to devel-docs/.