infra: set up CI workflows

[marc0olo]

Context

CI workflows from restructuring-attempt-1 need to be restored and updated for the current repository state.

Workflows to restore (from restructuring-attempt-1/.github/workflows/)

• Build check on every PR
• Validation scripts (frontmatter, no-dfx, no-mdx) — depends on infra: restore validation scripts #114
• Deployment to beta-docs.internetcomputer.org on push to main (may already exist via existing infra — verify)
• Link checking (internal links)

Steps

1. Check out restructuring-attempt-1 to review workflow files
2. Update workflow paths and scripts for current structure
3. Add required repository secrets documentation in README
4. Enable branch protection rules requiring CI to pass (see also infra: content gap analysis #118)