Move docs workflows to elastic/docs-actions

[Mpdreamz]

Summary

Moves our docs CI/CD workflow logic from elastic/docs-builder to
elastic/docs-actions.
elastic/docs-builder has grown in scope; elastic/docs-actions lets the
docs team push updates to the @v1 tag independently without coupling to
the broader builder tooling.

The new setup follows GitHub's recommended two-phase workflow approach:
all required checks on pull requests now run with read-only access only.
Preview deployments and PR comments happen in a separate phase triggered
after the required check completes.

Replaces preview-build.yml and docs-cleanup.yml with three new workflow
files: docs-build.yml, docs-deploy.yml, and docs-preview-cleanup.yml.

Preview deployments for forks:
Preview deployments are not available for fork PRs by default. If you are an
Elastic employee working from a fork, previews will work once your Elastic
GitHub organization membership is set to Public:

1. Go to https://github.com/orgs/elastic/people and find your name

2. Open the visibility dropdown next to your name and select Public

   Or: GitHub profile -> Organizations -> Elastic logo -> set visibility
   to Public.

Contributors outside Elastic will still get build validation feedback on
their PRs -- only preview deployments are skipped.

Post-merge

• Update branch protection rules to require docs-build instead of
  preview-build (if applicable)

cc @elastic/ingest-docs