Move `core-developers/` to `core-team/`

[hugovk]

We can already incrementally make some of the contrib changes to the main guide (see for example https://github.com/python/devguide/pull/1467 and https://github.com/python/devguide/pull/1461 for earlier work).

To better follow the language of PEP 13, prefer "core team" over "core developers".

This PR:

• Moves the core-developers/ files/URLs to core-team/
• Update and add redirects in conf.py so the old core-developers/ URLs go to core-team/
• Rename 'Core developers' section to 'Core team'
• Rename 'How to become a core developer' page to 'How to join the core team' and become-core-developer.rst to join-team.rst

Other prose changes can follow.

cc @nedbat @willingc

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📚 Documentation preview 📚: https://cpython-devguide--1593.org.readthedocs.build/

[hugovk]

And I think I'll rename:

• core-team/developer-log.rst to core-team/team-log.rst
• core-team/developers.csv -> core-team/core-team.csv

[willingc]

Love this @hugovk 🙏🏼