refactor(pypi): rename config settings and improve docs

[aignas]

Before the PR the config_setting names where following an internal
logic and those names would be leaking into the error messages when no
match is found. I thought that the names thus should be improved and
maybe made more similar to the whl filename parts that they are
derived from.

As part of this change I have also added more docs and added them to
sphinxdocs in the hopes that this documentation helps maintainers and
users looking at error messages alike.

Summary:

• Make names more similar to the whl filenames.
• Instead of having osx_<cpu>_universal2 config settings for each
  cpu value, have a single osx_universal2 config setting.
• Stop creating redundant/unused config settings
• Refactor the _dist_config_setting code to be simpler and create
  fewer targets by using a clever trick for the whl config setting
  flag value usage.

The stats:

$ bazel query //tests/pypi/config_settings/... | rg ":(|_)is" | wc -l
2223
$ bazel query @dev_pip//_config/... | wc -l
1982

$ bazel query //tests/pypi/config_settings/... | rg ":(|_)is" | wc -l
1780
$ bazel query @dev_pip//_config/... | wc -l
1066

Work towards #260