ci(pages): stop the gh-pages workflow from overwriting the docs site

[ArangoGutierrez]

The gh-pages branch is kept as an archive of the retired CI dashboard (#695).
Its .github/workflows/deploy.yaml still triggers on push to gh-pages and
on workflow_dispatch, and it calls actions/deploy-pages, which replaces the
entire Pages site.

The scheduled 6-hourly trigger was removed in #695, so nothing clobbers the
docs automatically. But a push to gh-pages or a manual dispatch would still
overwrite the Mokka documentation with the old dashboard, and the docs workflow
would not restore it until the next docs/** change lands on main.

Options, roughly in order of preference:

1. Remove the on: triggers from deploy.yaml on the gh-pages branch.
2. Disable the workflow with gh workflow disable.
3. Delete the branch entirely and rely on merge-commit history for recovery.

Archived trend data (history.json, issues_prs.json and four others) was
saved off the live site before the cutover and is not stored in the repo.