Document metadata consistency expectation (#15683)
An opinionated write-up on why Python packaging needs metadata
consistency, and that we need to extend metadata to accommodate ML and
scientific users.
I didn't add a paragraph related to CUDA or accelerators in general and
wheel variants, as this is currently support neither by wheel tags nor
by PEP 508 markers, so it's not a strict metadata consistency concern,
plus this would get outdated quickly as wheel variants progress.
