Migrate docs deployment to Pages artifacts on main

[kellyguo11]

Description

Backport #6571 from develop to main so the production documentation
workflow can deploy through GitHub Pages artifacts.

The current nightly workflow commits the complete generated multi-version
site to gh-pages. That branch contributes approximately 656 MiB of
compressed Git data and accounts for most of the repository clone payload.
This change uploads the generated site with actions/upload-pages-artifact
and publishes it with actions/deploy-pages instead.

Documentation content, URLs, and multi-version behavior are unchanged. After
this merges, a repository administrator must set the Pages source to GitHub
Actions, run the Docs workflow once from main, verify the deployment, and
then delete gh-pages after the rollback window.

Type of change

• Bug fix (non-breaking CI/infrastructure change)

Validation

• Confirmed the applied patch ID exactly matches merged PR Migrate docs deployment to Pages artifacts #6571.
• Ran ./isaaclab.sh -f before committing; all hooks passed.
• Ran ./isaaclab.sh -f again after committing and before pushing; all hooks
  passed.
• Validated the production-like workflow in
  https://github.com/kellyguo11/IsaacLab-public/actions/runs/29675383308.
• Verified the deployed root redirect, version selector, representative
  branches/tags, and static assets.

Checklist

• I have read and understood the contribution guidelines.
• I have run the pre-commit checks with ./isaaclab.sh -f.
• Documentation changes are not required; published content is unchanged.
• My changes generate no new warnings.
• A code test is not applicable; the deployment workflow was validated
  end-to-end in the fork.
• A package changelog fragment is not required because no package changed.
• The contributors from Migrate docs deployment to Pages artifacts #6571 are already represented in the project.

[greptile-apps]

Greptile Summary

This PR migrates the production docs deployment from pushing to a gh-pages branch (via peaceiris/actions-gh-pages) to the native GitHub Actions Pages artifact flow (actions/upload-pages-artifact@v4 + actions/deploy-pages@v4), eliminating the ~656 MiB compressed gh-pages branch from the repository's clone payload.

• The build-multi-docs job now uploads the generated site as a github-pages artifact instead of a named docs-html artifact, and the separate download step is removed.
• The deploy-docs job is simplified to a single actions/deploy-pages@v4 step with correctly scoped pages: write / id-token: write permissions and a github-pages environment block.
• An operational step is required post-merge: a repository administrator must switch the Pages source to "GitHub Actions" in repository settings before the next deploy run.

Confidence Score: 5/5

Safe to merge after an administrator switches the Pages source to "GitHub Actions" in repository settings.

The change is a straightforward, well-understood migration to the canonical GitHub-native Pages deployment pattern. Permissions are correctly scoped to the deploy job only, the github-pages environment name is exact, and the author validated the full deployment end-to-end in a fork. The build-latest-docs path (PRs and non-deploy branch pushes) is untouched and continues to work as before.

No files require special attention; the single changed file is a focused, minimal CI workflow update.

Important Files Changed

Filename	Overview
.github/workflows/docs.yaml	Replaces third-party peaceiris/actions-gh-pages push-to-branch deployment with the native actions/upload-pages-artifact + actions/deploy-pages Pages-artifact pipeline; adds proper job-level pages: write and id-token: write permissions and a github-pages environment block on the deploy job.

_{Reviews (1): Last reviewed commit: "Migrate docs deployment to Pages artifac..." | Re-trigger Greptile}