2.15.0: 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 build/sphinx/man
Running Sphinx v7.1.2
WARNING: Invalid configuration value found: 'language = None'. Update your configuration to a valid language code. Falling back to 'en' (English).

Traceback (most recent call last):
  File "/usr/lib/python3.8/site-packages/sphinx/registry.py", line 441, in load_extension
    mod = import_module(extname)
  File "/usr/lib64/python3.8/importlib/__init__.py", line 127, in import_module
    return _bootstrap._gcd_import(name[level:], package, level)
  File "<frozen importlib._bootstrap>", line 1014, in _gcd_import
  File "<frozen importlib._bootstrap>", line 991, in _find_and_load
  File "<frozen importlib._bootstrap>", line 973, in _find_and_load_unlocked
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 229, in __init__
    self.setup_extension(extension)
  File "/usr/lib/python3.8/site-packages/sphinx/application.py", line 402, in setup_extension
    self.registry.load_extension(self, extname)
  File "/usr/lib/python3.8/site-packages/sphinx/registry.py", line 444, in load_extension
    raise ExtensionError(__('Could not import extension %s') % extname,
sphinx.errors.ExtensionError: Could not import extension recommonmark (exception: No module named 'recommonmark')

Extension error:
Could not import extension recommonmark (exception: No module named 'recommonmark')