docs: Add sphinx API docs

[crandmck]

Ths PR adds a GH worfkow based on https://github.com/creator-assertions/creator-assertions.github.io/blob/main/.github/workflows/publish.yml, which will build the docs using Sphinx and deploy them to GH Pages for consumption. We can then link to that from the OS doc site.

Temp online preview of generated docs: https://crandmck.github.io/c2pa-python-docs/

Note:

• The script scripts/generate_api_docs.py generates docs into the api-docs/_build/html directory (to separate them from the handwritten markdown docs).
• The Sphinx configuration is in api-docs/conf.py
• Info on the API docs is in docs/project-contributions.md since (once we set up the automatic doc generation workflow) most users won't need to know how to generate the docs, unless contributing to the project.

[tmathern]

@crandmck I configured the repo so you can test publish from this branch. Once you merge, let me know and I'll remove the config for this specific branch (and keep only main). It is also configured to run this workflow from the main branch when a Github release is created. I had test deployments that worked, see https://contentauth.github.io/c2pa-python/. Feel free to merge when you are happy with how the docs look. (Note that the branch will need to be up-to-date with latest main to allow merging).