[1.6.x] Fixed #22813 -- Updated Pillow URL in documentation

Backport of dfa3505ff1 from master.
[1.6.x] Fixed #22818 -- Clarified you need to cd into the Django clone.

Thanks Josh Parris.

Backport of bcc3d2b978 from master
[1.6.x] Fixed #22349 -- Added a note clarifying `RawQuerySet` has no `__len__`.

Thanks cdestigter for the report.

Backport of 2d425116e2 from master