Document API release process and rollout steps

[Flo4604]

Capture the current API release process so it’s easy to follow:

• In unkey repo: git switch main, git pull, git tag v2.0.XX (latest +1), git push --tags.
• Wait for the release build to complete.
• In infra repo: update pulumi/projects/api/main.go to reference the new tag, open a PR and get it approved.
• Note: SDK updates via Speakeasy can be triggered manually in the SDK repo’s Actions tab.

[linear[bot]]

ENG-2209