Python PyPI deploy setup

[bnprks]

This is a large set of changes, described below with a summary of per-file changes. The end result is that can build and deploy a very basic BPCells package to PyPI across Mac, Linux and Windows (and ensure tests pass before deploy). There are also several docs updates to help add a Python section to the documentation website

Deploy to PyPI:

• .github/workflows/pypi.yml (lots of changes done after using a test repo to figure out much of the needed changes without waiting for slow build times)
• python/pyproject.toml Move all the cibuildwheel config into the pypi.yml workflow, since I felt a lot of that workflow pulled in specifics about the github CI environment
• python/scripts/install_*.sh Helper scripts for CI to install dependencies from source (or ccache from binary)
• python/setup.py Switch to a more flexible way of getting additional compiler arguments from environment variables (just CPATH and LIBRARY_PATH and removing HWY_* flags). Also fix up the logic for restoring messed up symlinks on windows checkouts, to include the new vendor folder

Updated docs:

• DEVELOPING.md Update instructions for generating docs website
• README.md Make parallel changes to r/index.md, though slight changes given that this is the github landing page
• python/DEVELOP.md (Takes most of the old README content and adds a section on the CI setup)
• python/README.md (Trying to be a short README as the PyPI landing page)
• python/docs/source/index.md , python/docs/source/python.md Try to mostly unify these two pages -- they are only separate due to the PyData Sphinx theme setting the top nav bar based on the table of contents of the landing page
• r/index.md Hide the detailed compilation instructions behind a <details> dropdown, and add a call-out to the Python docs site up top
• r/pkgdown/_pkgdown.yml, r/pkgdown/extra.css. Add a link to the Python docs in the website header, with a slight CSS styling adjustment to make it not too big

Misc changes:

• python/src/fragments.cpp: One of the tests failed on Windows, due to temporary files still being open at the time of deletion in a C++ function. (Windows is a bit unique in that it disallows deleting open files). Just added in a line to cause the appropriate C++ destructor to be called to close temp files prior to deleting them
• r/src/bpcells-cpp/arrayIO/binaryfile.h: Add a quick compatibility check for the Windows MSVC compiler to call the correct name for the builtin 32-bit byteswap function.

[bnprks]

I consolidated to a single linux/mac script to build dependencies from source, then switched to recommending that in DEVELOP.md for a simpler (albeit slower) setup process.

Also made a few changes in the actions workflow to change how caching the dependencies would work, and after much trial and error it seems to be functional again.