ci: migrate PR website deploy from Azure Blob Storage to GitHub Pages

[Hotell]

NOTE:

Initial plan was to migrate to https://github.com/actions/upload-artifact/releases/tag/v7.0.0 but unfortunately they support only single file for now. will revisit once their support expands to multi file upload

Description

Migrates the PR preview website deployment infrastructure from Azure Blob Storage to GitHub Pages, eliminating the Azure dependency and using only GitHub-native infrastructure.

Changes

• docsite-publish-ghpages.yml — Replaced actions/upload-pages-artifact + actions/deploy-pages with direct git push to gh-pages branch. Uses clean-exclude pattern to preserve pr-preview/ directory during production deploys.
• pr-website-deploy-comment.yml — Removed azure/login, azure/cli actions and Azure env vars. Deploys PR preview to gh-pages branch under pr-preview/pr-NUMBER/ via git CLI.
• pr-website-cleanup.yml (new) — Cleans up PR preview directory from gh-pages branch on pull_request_target: closed.
• pr-deploy-site.js — Updated updatePrOrBranchLink() URL parsing to recognize the new /pr-preview/pr-NUMBER/ path format.

URL Change

	URL
Before	https://fluentuipr.z22.web.core.windows.net/pull/NUMBER/
After	https://microsoft.github.io/fluentui/pr-preview/pr-NUMBER/

Security

• Zero third-party actions for deployment — uses only actions/checkout + git CLI
• Azure secrets (AZURE_CLIENT_ID, AZURE_TENANT_ID, AZURE_SUBSCRIPTION_ID) can be removed after merge
• Fork PR support maintained via workflow_run + pull_request_target triggers

Manual prerequisite (repo admin)

Settings → Pages → Source must be changed from "GitHub Actions" to "Deploy from branch" (gh-pages / / (root)) before the new workflows will work.

Co-authored-by: Copilot 223556219+Copilot@users.noreply.github.com

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