docfx: sync from repo-template (gh-pages bootstrap fix)

[Chris-Wolfgang]

Summary

Sync .github/workflows/docfx.yaml from repo-template to pick up the gh-pages bootstrap fix from Chris-Wolfgang/repo-template#337.

What changed

The "Deploy docs to GitHub Pages" step had a latent bug: when the gh-pages branch did not yet exist on the remote, $WORK_DIR was created as a plain directory and the subsequent git add / diff --cached / commit / push failed with fatal: not a git repository. The fix initializes the directory as a git repo on gh-pages and adds the authenticated origin remote, so the existing add/commit/push works on a first-ever deploy. Cleanup also branches between git worktree remove (worktree path) and Remove-Item (fresh-repo path).

If gh-pages already exists in this repo (the common case), this change is a no-op at runtime — the bootstrap branch is only taken on a fresh repo.

Test plan

• Workflow YAML lints / next docs deploy succeeds
• (Optional) verify gh-pages is unaffected

[Copilot]

Pull request overview

Note

Copilot was unable to run its full agentic suite in this review.

Syncs the DocFX GitHub Actions workflow with the repository template to fix first-time gh-pages deployments when the branch doesn’t exist yet.

Changes:

• Detects whether gh-pages exists and chooses between a worktree-based flow vs. a fresh repo bootstrap.
• Bootstraps a new git repository in $WORK_DIR on gh-pages (including an origin remote) for first-time deploys.
• Adjusts cleanup to remove either the worktree or the freshly-initialized repo directory, and resets $LASTEXITCODE.

[Chris-Wolfgang]

Superseded by the D8 canonical sync that just landed on main. The gh-pages bootstrap fix this PR was carrying ($branchExists = git ls-remote --heads origin gh-pages guard) is already present in main's docfx.yaml. Merging this PR would actually revert main's D8 canonical work back to the older docfx.yaml — closing instead.