4.0.1: documentation uses `recommonmark` which is now deprecated

[kloczek]

From https://github.com/readthedocs/recommonmark

Warning: recommonmark is now deprecated. We recommend using MyST for a docutils bridge going forward. See readthedocs/recommonmark#221 for background and discussion.

+ /usr/bin/sphinx-build -n -T -b man docs/source build/sphinx/man
Running Sphinx v7.1.2

Traceback (most recent call last):
  File "/usr/lib/python3.8/site-packages/sphinx/config.py", line 356, in eval_config_file
    exec(code, namespace)  # NoQA: S102
  File "/home/tkloczko/rpmbuild/BUILD/nbdime-4.0.1/docs/source/conf.py", line 19, in <module>
    import recommonmark.parser
ModuleNotFoundError: No module named 'recommonmark'

The above exception was the direct cause of the following exception:

Traceback (most recent call last):
  File "/usr/lib/python3.8/site-packages/sphinx/cmd/build.py", line 285, in build_main
    app = Sphinx(args.sourcedir, args.confdir, args.outputdir,
  File "/usr/lib/python3.8/site-packages/sphinx/application.py", line 207, in __init__
    self.config = Config.read(self.confdir, confoverrides or {}, self.tags)
  File "/usr/lib/python3.8/site-packages/sphinx/config.py", line 179, in read
    namespace = eval_config_file(filename, tags)
  File "/usr/lib/python3.8/site-packages/sphinx/config.py", line 369, in eval_config_file
    raise ConfigError(msg % traceback.format_exc()) from exc
sphinx.errors.ConfigError: There is a programmable error in your configuration file:

Traceback (most recent call last):
  File "/usr/lib/python3.8/site-packages/sphinx/config.py", line 356, in eval_config_file
    exec(code, namespace)  # NoQA: S102
  File "/home/tkloczko/rpmbuild/BUILD/nbdime-4.0.1/docs/source/conf.py", line 19, in <module>
    import recommonmark.parser
ModuleNotFoundError: No module named 'recommonmark'


Configuration error:
There is a programmable error in your configuration file:

Traceback (most recent call last):
  File "/usr/lib/python3.8/site-packages/sphinx/config.py", line 356, in eval_config_file
    exec(code, namespace)  # NoQA: S102
  File "/home/tkloczko/rpmbuild/BUILD/nbdime-4.0.1/docs/source/conf.py", line 19, in <module>
    import recommonmark.parser
ModuleNotFoundError: No module named 'recommonmark'