See https://stackoverflow.com/questions/74365771/copy-an-older-version-of-an-existing-file-into-a-seperate-file-which-preserves-h

* Rudimentary `array_api_stubs` pkg and folder-per-version docs
* Regex nitpick ignore for array types
* Move `2021.12` docs to respective sub-folders
* Fix autosummary by patching stub sub-modules in `conf.py`
* Rudimentary build-all Makefile rule
* Specify versions in `conf.py`
* Use `versions.json` method for sphinx-material

  Otherwise don't think we can get links working on nested pages

  Notably this usually won't work locally - see
  https://developer.mozilla.org/en-US/docs/Web/HTTP/CORS/Errors/CORSRequestNotHttp

* Update image paths to shared `_static/`
* Use vars in `make build` definition
* Expand regex used to identify unwanted type hint prefixes
* Update `pages.yml` workflow
* Store gh-pages `.gitignore` in `main` to be copied over
* `_spec_conf.py` -> `_array_api_conf.py`
* Basic package necessities
* Replace `requirements.txt` with extra dependency `doc`
* Basic local build guide
* Update preview workflow
* Remove out-of-date make things, new top-level Makefile
* Minor `CONTRIBUTING.md` improvements
* Basic `_array_api_conf.py` module docstring
* Move quickstart to `README.md`
* Use `array_api` as opposed to `signatures` for `2021.12` autodoc
* More consistent Makefile
* Introduce `_ghpages` to hold the non-Sphinx static files
* Rudimentary spec release guide
* Guide to preserve history for future spec releases