Edit rules for creating and removing gitlab pages

- fix the issue with feature branches landing under main path which was
  introduced with !768
  (see e.g. https://gitlab.syncad.com/hive/clive/-/jobs/2163981#L103)
- place pages of master branch under `stable/` instead of default
  (develop) branch under `default/`, also redirect to `stable/`
- store pages for protected tags under `tags/` path
- store pages for protected branches under `protected/` path
- move rules out of docs_variables to avoid mistakes - adding `when`
  keyword in this place would also affect `build_docs` and `clean_docs` jobs
- disallow removing pages for master branch, protected tags and
  protected branches