add basic sphinx docs build using apidoc

[sarnold]

• update .gitignore and .gitchangelog.rc and (re)generate new changelog
• add sphinx docs build using apidoc extension and readme/changelog symlinks
• rst apidoc modules are auto-generated and are in .gitignore along with the generated html dir
• add dependencies to packaging and add docs/changes cmds to tox file. Includes a tox extension for shared tox environments; the new tox commands are an example of this => 4 cmds using one tox env.

[sarnold]

Note my usual approach to docs is a workflow to build/push docs to gh-pages branch for pull-requests, and "release" docs in the release workflow. You should probably make the orphan gh-pages branch as shown here. So the sphinx workflow creates html artifacts and deploys to gh-pages while the docs section of release workflow pushes docs with a release version.

[sarnold]

aaaand I did not want this to fall through the giant PR cracks, so the package versioning bits are now using setuptools_scm so after building locally it looks like this:

$ pip install pyre2 --pre --force-reinstall --prefer-binary -f dist/
Looking in links: dist/
Processing ./dist/pyre2-0.3.7.dev37+gba4931b.d20240911-cp311-cp311-linux_x86_64.whl
Installing collected packages: pyre2
  Attempting uninstall: pyre2
    Found existing installation: pyre2 0.3.6
    Uninstalling pyre2-0.3.6:
      Successfully uninstalled pyre2-0.3.6
Successfully installed pyre2-0.3.7.dev37+gba4931b.d20240911