Adding cmake functions to generate docs

[leakec]

Implementing #23.

• Added a function as described in the issue with the addition of EXTRA_ARGS.
• Added cmake config and version so one can do find_package(pybind11_mkdoc)
• Added get_cmake_dir function to easily get the CMake directory.
• Added unit tests.
• Added docs to the README.md.

[leakec]

Tagging @henryiii for a review.

EDIT: No longer need help with what is below, I got this all sorted.
and some help with the CI. I have not used uv run at all (I've only used it for venv and installing packages). I'd like to ensure pybind11 is installed in the environment the tests are running in, as cmake will be searching for it. However, even though it is in the test group it does not seem to install automatically. I also added it to the --with statement, but that didn't seem to do it either.