[4.2.x] Refs #34140 -- Added configurations to run blacken-docs linter and adjusted docs.

This adds:
- GitHub actions,
- tox configuration,
- pre-commit hook, and
- makefile rules
to run blacken-docs linter.

Co-authored-by: David Smith <smithdc@gmail.com>

Backport of 6015bab80e from main

docs/internals/contributing/writing-documentation.txt

@@ -211,6 +211,9 @@ documentation:
   "last bit" of that path. So ``:mod:`~django.contrib.auth``` will
   display a link with the title "auth".
 
+* All Python code blocks should be formatted using the `blacken-docs`_
+  auto-formatter. This will be run by ``pre-commit`` if that is configured.
+
 * Use :mod:`~sphinx.ext.intersphinx` to reference Python's and Sphinx'
   documentation.
 
@@ -258,6 +261,13 @@ documentation:
   also need to define a reference to the documentation for that environment
   variable using :rst:dir:`.. envvar:: <envvar>`.
 
+.. versionchanged:: 4.2
+
+    All Python code blocks in the Django documentation were reformatted with
+    `blacken-docs`_.
+
+.. _blacken-docs: https://pypi.org/project/blacken-docs/
+
 Django-specific markup
 ======================