Document the release and promote actions

[stijnvanhulle]

Summary

• Add release and promote actions for the shared changesets release flow #20 added .github/actions/release and .github/actions/promote but never updated the README, which still only documented .github/setup.
• Adds a ### .github/actions/release and `### `.github/actions/promote section for each, in the same style as the existing .github/setup section: a short description, an Inputs table, an Outputs table, and a Usage example.
• Inputs/outputs/defaults are copied from the actual action.yml files so the README stays in sync with the code.

Docs-only change, no behavior affected.

Test plan

• Confirmed only README.md changed.
• All 5 fenced yaml code blocks in the README parse as valid YAML.

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Latest commit: 9ac72e2

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