Fix PR doc previews being wiped on every main branch push

[Copilot]

Every push to main force-replaced the entire gh-pages branch with only the freshly built main docs, deleting all pr-*/ preview directories and breaking any preview URLs already posted in PR comments.

Change

• Added keep_files: true to the Deploy Docs step (main branch deploy) in docs.yml, mirroring what the PR preview deploy step already had

# Before
- name: Deploy Docs
  uses: peaceiris/actions-gh-pages@v3
  with:
    github_token: ${{ secrets.GITHUB_TOKEN }}
    publish_dir: doc/_build/html

# After
- name: Deploy Docs
  uses: peaceiris/actions-gh-pages@v3
  with:
    github_token: ${{ secrets.GITHUB_TOKEN }}
    publish_dir: doc/_build/html
    keep_files: true        # preserves pr-*/ preview dirs on gh-pages

The existing docs-pr-cleanup.yml workflow already handles removal of stale preview directories when PRs are closed, so accumulated pr-*/ dirs are not a concern.

---

📍 Connect Copilot coding agent with Jira, Azure Boards or Linear to delegate work to Copilot in one click without leaving your project management tool.