ci: revert PR #322 and fix docs deploy with github-pages environment

[fsamier]

Summary

Reverts PR #322 and applies a minimal fix instead: the docs deployment was timing out on main because the deploy-docs job never declared the github-pages environment, so the Pages deployment stayed stuck in deployment_queued and failed after 10 minutes.

Changes

• Revert PR ci: build and deploy docs only on push to main #322 ("Build and deploy docs only on push to main"), restoring the original build-once / upload-artifact / download-and-deploy flow.
• Add the github-pages environment (with url: ${{ steps.deployment.outputs.page_url }}) to the deploy-docs job and an id: deployment on the deploy step. actions/deploy-pages requires the job to target this environment; without it the deployment never resolves and times out — the root cause of the failure.

Result

Docs are built once in ci, the artifact is uploaded only on push to main, and the deploy-docs job now deploys to GitHub Pages successfully instead of hanging.