Fixed #20286 - Typo in static files docs.

Thanks bmispelon.
Fixed #20285 - Added missing commas in translation docs.

Thanks cody.j.b.scott@
Complemented documentation following commit be9ae693c

Refs #17840. Thanks Carl Meyer for noticing the omission.
Explained the pattern for special methods compatibility.
Fixed #20276 -- Implemented __bool__ for MergeDict

MergeDict evaluates now to False if all contained dicts are empty.
Thanks til for the report and the initial patch.
Fix != operations on lazy objects.
Updated tutorial INSTALLED_APPS section (removed contrib.sites)
Fixed #20104 -- Changed VersionDirective in order to avoid ambiguity.

As explained in ticket #20104, the use of versionchanged/versionadded
was confusing.

To solve this ambiguity these directives no longer accept a second
argument but now they only receive the version number (1st arg) and then
a content with the proper comment.
Removed unused imports.
Adapted uses of versionchanged/versionadded to the new form.

Refs #20104.
Added 'polls' sub directory under static and reorganized to more accurately represent the project; compared using 'tree' command.
Merge pull request #1024 from jktravis/master

Updated reusable apps doc's review of project layout
Fixed #19211 -- Adapted tutorial for Python 3
Fixed #20270 -- Fixed error in AjaxResponseMixin documentation
Fixed #20307 -- Word repeated in tutorial.

Replaced "the the" in the tutorial with "the".
Fixed the reusable apps docs to use a rst file extension for the README and follow PEP8 a bit more.
Fixed a rST bug in the README template in the reusable apps docs.
Fixed documentation of disable_constraint_checking

The docstring and base implementation of disable_constraint_checking do not indicate that a return value is expected for proper behavior.
Fix CharField typo in legacy-databases.txt docs

The example Person model in the legacy databases HowTo had a typo referring to ChaField instead of Charfield
Merge pull request #1029 from carmi/patch-1

Fix CharField typo in legacy-databases.txt docs