ci: build and deploy docs only on push to main

[fsamier]

Summary

Fixes the deploy docs job that was timing out on main (the Pages deployment stayed stuck in deployment_queued and eventually failed after 10 minutes).

Changes

• Build docs on every PR (in the ci job) to catch build errors early, without uploading any artifact.
• Made deploy-docs a self-contained job that builds and deploys docs, gated on github.event_name == 'push' && github.ref == 'refs/heads/main'.
• Declared the github-pages environment on the deploy job, which actions/deploy-pages requires. Without it the deployment never resolves and times out — the root cause of the failure.

Result

Docs are built on every PR to anticipate errors, and built + deployed to GitHub Pages only on push to main, where the Pages deploy step now resolves correctly instead of hanging.

[fsamier]

Updated: docs are now built on every PR push (no artifact upload) to catch build errors early, while build + deploy to GitHub Pages still runs only on push to main.

[fsamier]

Refactored: the deploy step now lives inside the main ci job and only runs on push to main (via an if condition). The separate deploy-docs job was removed.

[fsamier]

Reverted to the two-job implementation: docs are built on every PR in ci, and a separate deploy-docs job builds + deploys to GitHub Pages only on push to main.