ci: Add `docs-preview.yml` to preview documentation builds when PR contains `docs` label

[soheeeeP]

issue #619

• Description

  • Add workflow to preview documentation builds which is executed only when a PR contains docs label.

• Process

  1. Update a project and fetch activate/deactivate versions
  2. Activate PR branch: update a version_slug of its project
  3. Trigger a new build for the version_slug
  4. Deactivate PR branch: update a version_slug of its project

• Question

  • I have a question about RTD build process. I wonder if when the English version of the documentation is successfully build/served, the process for Korean version documentation will be automatically triggered or it should be build/served manually.

[soheeeeP]

Can anyone assign docs label to this PR?

[soheeeeP]

@achimnol RTD api requests fails with 401(unauthorized) status code. It seems something went wrong with repository secret(RTD_AUTH_TOKEN).

[achimnol]

Hm... I think secret access is not allowed from PRs by forks. https://github.com/openmainframeproject/feilong/issues/367 Let me think how we could workaround it.

[achimnol]

Currently this is being handled by the updated RTD webhooks.