Add a contributing how-to in the repo

[badlydrawnrob]

For the documentation (although you may not want others to fork this?) similar to the Piccolo ORM one, with requirements (which I've managed to do) and ./scripts/run-docs.sh (which isn't there and can't get uv run serve_docs.py to work).

Also getting the following error with cd docs/ && make html:

Running Sphinx v7.3.7

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

Traceback (most recent call last):
  File "/Users/name/Sites/GitHub/piccolo_admin/.venv/lib/python3.13/site-packages/sphinx/config.py", line 509, in eval_config_file
    exec(code, namespace)  # NoQA: S102
    ~~~~^^^^^^^^^^^^^^^^^
  File "/Users/name/Sites/GitHub/piccolo_admin/docs/source/conf.py", line 29, in <module>
    import piccolo_admin  # noqa
    ^^^^^^^^^^^^^^^^^^^^
  File "/Users/name/Sites/GitHub/piccolo_admin/piccolo_admin/__init__.py", line 1, in <module>
    from .endpoints import FormConfig, create_admin
  File "/Users/name/Sites/GitHub/piccolo_admin/piccolo_admin/endpoints.py", line 19, in <module>
    import typing_extensions
ModuleNotFoundError: No module named 'typing_extensions'

make: *** [html] Error 2

[sinisaos]

@badlydrawnrob You need to do next things:

1. run pip install -r requirements/readthedocs-requirements.txt (to install all dependencies)
2. cd docs
3. make html
4. run python serve_docs.py

I will make PR to add that to docs.

[sinisaos]

Completed in #461