[analyzer] Move alpha checker EnumCastOutOfRange to optin (#67157)

The checker EnumCastOutOfRange verifies the (helpful, but not
standard-mandated) design rule that integer to enum casts should not
produce values that don't have a corresponding enumerator. As it was
improved and cleaned up by recent changes, this commit renames it from
`alpha.cplusplus.EnumCastOutOfRange` to `optin.core.EnumCastOutOfRange`
to reflect that it's no longer alpha quality.

As this checker handles a basic language feature (which is also present
in plain C), I moved it to a "core" subpackage within "optin".

In addition to the renaming, this commit cleans up the documentation in
`checkers.rst` and adds the new example code to a test file to ensure
that it's indeed producing the behavior claimend in the documentation.

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