Split supported architectures by base in the documentation

[mr-cal]

What needs to get done

• 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.
• Update outdated docs or point them to read the docs: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

Why it needs to get done

To make it clear which architectures are supported for which bases.

[syncronize-issues-to-jira]

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The internal ticket has been created: https://warthogs.atlassian.net/browse/CRAFT-4047.

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