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>

docs/internals/contributing/writing-code/submitting-patches.txt

@@ -291,9 +291,9 @@ All code changes
 
 * Does the :doc:`coding style
   </internals/contributing/writing-code/coding-style>` conform to our
-  guidelines? Are there any  ``black``, ``flake8``, or ``isort`` errors? You
-  can install the :ref:`pre-commit <coding-style-pre-commit>` hooks to
-  automatically catch these errors.
+  guidelines? Are there any  ``black``, ``blacken-docs``, ``flake8``, or
+  ``isort`` errors? You can install the :ref:`pre-commit
+  <coding-style-pre-commit>` hooks to automatically catch these errors.
 * If the change is backwards incompatible in any way, is there a note
   in the release notes (``docs/releases/A.B.txt``)?
 * Is Django's test suite passing?

docs/internals/contributing/writing-code/unit-tests.txt

@@ -69,17 +69,18 @@ command from any place in the Django source tree:
     $ tox
 
 By default, ``tox`` runs the test suite with the bundled test settings file for
-SQLite, ``black``, ``flake8``, ``isort``, and the documentation spelling
-checker. In addition to the system dependencies noted elsewhere in this
-documentation, the command ``python3`` must be on your path and linked to the
-appropriate version of Python. A list of default environments can be seen as
-follows:
+SQLite, ``black``, ``blacken-docs``, ``flake8``, ``isort``, and the
+documentation spelling checker. In addition to the system dependencies noted
+elsewhere in this documentation, the command ``python3`` must be on your path
+and linked to the appropriate version of Python. A list of default environments
+can be seen as follows:
 
 .. console::
 
     $ tox -l
     py3
     black
+    blacken-docs
     flake8>=3.7.0
     docs
     isort>=5.1.0