Improve the "internals" docs

[Aymeric Augustin]

I proof-read all docs/internals, fixing typos, updating outdated links, adding some headers, and rewriting paragraphs that weren't consistent — either internally or with the current state of things.

Here are the notable changes:

• I added a paragraph to highlight the fact that producing code is not the only way to contribute to Django. Helping other users is also very important, as it frees up core developers time.

• I documented the new backport policy, as described in Jacob's email a few weeks ago.

• I rewrote unrealistic backwards-compatibility claims.

• I consistently recommended to ask for new features on django-developers: in some places, I found sentences that boiled down to "ask for new features on Trac", although the paragraph dedicated to new features says to ask of the mailing list first.

• I suggested to check for unused imports before submitting a patch, to reduce the need for r16539 style cleanup.

Note to reviewers: I re-indented deprecation.txt entirely to add navigable headers, but I didn't touch its content at all.

[Julien Phalip]

It looks pretty awesome to me.

[Jannis Leidel]

In [16548]:

(The changeset message doesn't reference this ticket)