Align Python release process with immutable tag rules

[daviburg]

What

Aligns the Python SDK release docs and ADO release template with the immutable-tag release process.

Why

We are preparing to add tag/release protection rules for Python after owner confirmation:

• numeric/PEP440-style tag allowlist (Python uses bare numeric tags, not v*)
• immutable release tag protection (deletion + non_fast_forward)

The current docs reference a non-existent .github/workflows/release.yml and recommend deleting/recreating failed release tags. The actual flow is Azure DevOps eng/ci/library-release.yml, which creates release/{version} and bare numeric tags, then creates a GitHub draft release and hands off to the partner PyPI release flow.

Changes

• Documents the actual ADO release pipeline flow.
• Removes the delete/recreate-tag retry path.
• Recommends retrying with a new version, e.g. 1.2.3.post1.
• Updates the ADO version regex to accept .postN versions so the documented retry path works.

Validation

• Regex accepts 1.2.3, 1.2.3a1, 1.2.3b1, 1.2.3rc1, 1.2.3dev1, 1.2.3.post1.
• Regex rejects v1.2.3 and 1.2.3-preview.1.
• git diff --check passes.

-- Dobby