docs: split supported CPU architectures

[AntoninKns]

Fixes #5224

• Split the reference documentation about supported architectures by base.
  • The difference between bases is that support for i386 and powerpc (not ppc64el) was dropped in core22 and newer bases.
  • "Supported locally" and "supported by launchpad" are the same for core22 and newer bases. For core20, "powerpc" is locally supported.

Support for core20 and core22 and higher is now correctly split according to the request.

• Update outdated docs or point them to read the docs:
  • https://snapcraft.io/docs/reference-architectures#supported-architectures
  • https://snapcraft.io/docs/build-from-github

The links above were broken and what seemed to be the corresponding doc now was already pointing to the supported architectures.

I spotted this issue that looked like a good first issue, I stay available for any further modifications.
I believe the meta description update is not relevant in this case and I suppose that I should wait for the approval of this PR to update the relevant release notes.
Best regards.

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• I've followed the contribution guidelines.
• I've signed the CLA.
• I've successfully run make lint && make test.
• I've added or updated any relevant documentation.
• In documents I changed, I added a meta description if one was missing.
• I've updated the relevant release notes.