BUILD-11462: Add README for update-release-channel action

[jayadeep-km-sonarsource]

BUILD-11462

Adds update-release-channel/README.md documenting the action: inputs/outputs, the two adoption patterns (release-workflow follow-up job and standalone workflow_dispatch), the required GitHub Environment with reviewer gate, operational recipes (rollback, delayed promotion, backfill, channel migration), and the publish-vs-promote atomicity caveat with recovery procedure.

Also extends the channel allowlist to include dogfood, so latest | stable | beta | rc | dogfood are all accepted (script regex + action input description + README updated together). This is needed for the sonarqube-cli adoption (SonarSource/sonarqube-cli#336).

[hashicorp-vault-sonar-prod]

BUILD-11462

[Copilot]

Pull request overview

Adds end-user documentation for the update-release-channel GitHub Action and extends supported release channels to include dogfood, keeping the action’s validation and docs in sync.

Changes:

• Added update-release-channel/README.md documenting usage patterns, required permissions, and inputs/outputs.
• Extended the channel allowlist to accept dogfood (script validation + action input description + docs).
• Added a top-level README section linking to the action’s full reference and providing usage snippets.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 2 comments.

File	Description
update-release-channel/update-release-channel.sh	Extends channel validation regex to include dogfood.
update-release-channel/README.md	New detailed action documentation (requirements, usage patterns, inputs/outputs).
update-release-channel/action.yml	Updates the channel input description to include dogfood.
README.md	Adds the action to the repository index and documents it at the top level.

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[julien-carsique-sonarsource]

Why a duplicated README in update-release-channel?
The other sub-actions have no README. I would remove it, unless there is additional information for development for instance.

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues
0 New dependency risks

Measures
0 Security Hotspots
100.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud

[gitar-bot]

Code Review ✅ Approved

Documentation for the update-release-channel action has been added, and the allowed release channels extended to include 'dogfood'. No issues were found.

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