Replace failing docs push-sync with a release-branch pull for the website

### Task Summary
The `sync-docs-to-site.yml` workflow that pushes `docs/` into `apache/incubator-texera-site` has failed on every run: its token identity (`aicam`) has no write access to the site repo, so the published docs drifted stale. Replace the push model with a pull: the website pulls `docs/` from the `release/vX.Y` branches at build time, and this repo only fires a `repository_dispatch` to the site when a release candidate is cut.
### Task Type
- [x] DevOps / Deployment / CI

Start with the sync-docs-to-site.yml workflow and trace how it currently pushes docs/ to apache/incubator-texera-site. Check the release/vX.Y branch and release-candidate flow to define where repository_dispatch belongs. Done means the failing push model is removed, the site pulls docs/ at build time, and dispatches occur only when a release candidate is cut.

Written by the indexing model from the issue text.