Feat: Build Docs Preview on PR

[RobertRosca]

CI/CD changes:

• Manually run commands mkdocs gh-deploy would run to have more control
• docs workflow remains functionally the same
  • however instead of overwriting the gh-pages branch it removes all files, puts the new build in, restores the pr directory (containing PR previews), commits, pushes
• Add docs-pr workflows, which:
  • docs-pr.yml - builds docs like normal, puts them in a directory ./pr/{PR_NO} in the gh-pages branch, pushes, adds comment to PR w/ link to preview page
  • docs-pr-close.yml - removes directory from gh-pages branch, updates comment

Docs changes:

• Adds new page docs/maintenance/ci-cd.md describing ci/cd

[github-actions]

View preview here - https://European-XFEL.github.io/environments/pr/37 (build for 585a5a9)

Preview removed on merge/PR close with 274d2b2

[RobertRosca]

Seems to have worked:

• The new docs page - https://european-xfel.github.io/environments/pr/37/maintenance/ci-cd/
• For testing I did this PR on my fork - Feat/pr preview docs RobertRosca/environments#4
  • Initial docs preview - https://github.com/RobertRosca/environments/actions/runs/7707887926?pr=4
  • Updating on change - https://github.com/RobertRosca/environments/actions/runs/7707921854?pr=4
  • Cleanup on merge - https://github.com/RobertRosca/environments/actions/runs/7707929892?pr=4
  • Main docs re-build on merge - https://github.com/RobertRosca/environments/actions/runs/7707929973