Move tokenstorage

[derek-globus]

To help with reviewing, each of the below steps have been done in separate commits.

What?

1. Move TokenStorage and its subclasses from globus_sdk.experimental.tokenstorage to globus_sdk.tokenstorage (with respect to terms of where it's exposed).
2. Move the existing docs/tokenstorage.rst doc to docs/authorization/token_caching/storage_adapters.rst, leaving behind an orphaned page redirecting users with existing links.
3. Update docstrings and create the docs/authorization/token_caching/token_storage.rst page.

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📚 Documentation preview 📚: https://globus-sdk-python--1065.org.readthedocs.build/en/1065/

[derek-globus]

Internal Tracking

https://app.shortcut.com/globus/story/35753

[derek-globus]

Maybe controversially I titled this section "token caching" rather than "token storage". This is to disambiguate with the subpages that provide references for specific versions of the same concept.

Alternative Structure

docs/
  - authorization/
     - token_storage/
         - token_storages.rst
         - index.rst
         - storage_adapters.rst

and you'd end up with pages titled "Token Storage", "Token Storages", and "Storage Adapters (Legacy)".

[kurtmckee]

I acknowledge that there is quite a bit of docstring pedantry here.

Nevertheless, I think that it will increase internal consistency, and make the documentation more readable.