* docs: use moderncmakedomain, Sphinx 4

* docs: Fix CMake example syntax in FindPythonExtensions documentation

This commit fixes the following warning reported when building
the documentation:

  /path/to/scikit-build/skbuild/resources/cmake/FindPythonExtensions.cmake:169:
  WARNING: Could not lex literal_block as "cmake". Highlighting skipped.

* docs: Update "generators.rst" fixing documentation formatting issues

This commit fixes the following warnings reported when building
the documentation:

  /path/to/scikit-build/docs/generators.rst:130:
  WARNING: Inline strong start-string without end-string.

  /path/to/scikit-build/docs/generators.rst:223:
  WARNING: Block quote ends without a blank line; unexpected unindent.

  /path/to/scikit-build/docs/generators.rst:223:
  WARNING: Unexpected section title or transition.

Co-authored-by: Jean-Christophe Fillion-Robin <jchris.fillionr@kitware.com>