Docs: added cabal domain for sphinx

It features simple `pkg-section` directive for marking sections for
which subsequent `pkg-field` fields are defined. Needed to disambiguate
fields such as `type` or `main-is`.

Converted most of `developing-packages.rst` to use new directives

Future work:

* Add directives for describing project.local, make it reference
  relevant cabal arguments

* Add more meta, like since or deprecated which can be rendered in link
  titles and index.

* Add "quick reference" indices for cabal args, package.cabal and
 project.local fields. Reference these from sidebar.

* Using "since" and "deprectated field it is possible to create "what's
  new" index

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