Alpine Linux is distinct from Ubuntu/Debian because Alpine's standard…

[xuganyu96]

Update PLATFORMS.md to reflect a tier 2 support for Alpine Linux. The commit message briefly explains why Alpine Linux is supported, and why it is a tier 2 support (for now, though maybe extending constant-time tests to Alpine is easy, and we can quickly whip up the GitHub Action workflow for that, as well?).

This PR closes #2304.

• [NO] Does this PR change the input/output behaviour of a cryptographic algorithm (i.e., does it change known answer test values)? (If so, a version bump will be required from x.y.z to x.(y+1).0.)
• [NO] Does this PR change the list of algorithms available -- either adding, removing, or renaming? Does this PR otherwise change an API? (If so, PRs in fully supported downstream projects dependent on these, i.e., oqs-provider will also need to be ready for review and merge by the time this is merged. Also, make sure to update the list of algorithms in the continuous benchmarking files: .github/workflows/kem-bench.yml and sig-bench.yml)

[coveralls]

coverage: 83.588% (+0.001%) from 83.587%
when pulling 8812044 on gyx-2304-document-alpine-linux-support
into 46e2719 on main.