python.mk: rename PYTHON_EXT_SUFFIX to PYTHON_TAG, document, etc

EXT_SUFFIX, according to PEP 3149, refers to the full tag and
extension for compiled extension module objects, eg .cpython-311.so,
.cpython-313t.so, etc. We do not use the correct semantic meaning,
and this usage becomes ambiguous and confusing when tags do not
match between bytecode and compiled extension module objects.

Rename our PYTHON_EXT_SUFFIX to PYTHON_TAG to align with PEP 3147's
specification of a magic tag, which consists of implementation name
and shorthand version only. This is meant for bytecode and other
files containing this tag in their filename that do not depend on
a specific Python ABI for the same version. Chase all existing
consumers.

Introduce PYTHON_SOABI to align with PEP 3149's specification of a
tag, which consists of implementation name, shorthand version and
any ABI flags present (this and PYTHON_TAG are identical without
ABI flags). This is meant for compiled extension module objects and
other files that depend on a specific Python ABI for the same
version.

Add documentation for these variables that our PYTHON_EXT_SUFFIX
never had.

PR: 274671
Event: Kitchener-Waterloo Hackathon 202506