Add PR preview deploys for non-source branches

[ladavid]

Summary

Adds a second GitHub Actions workflow (.github/workflows/preview.yml) that builds the mkdocs site for any non-source branch and publishes it to main/previews/<branch>/, so reviewers can see a rendered preview before merge.

• Push to any branch other than source / main / gh-pages triggers a build and deploy to previews/<branch-slug>/ (slashes in branch names flatten to -).
• PR opened/synchronized against source posts a sticky comment with the preview URL and the head commit SHA; subsequent pushes update the same comment in place.
• PR closed/merged removes the preview subfolder from main and updates the sticky comment.
• keep_files: true ensures previews never clobber the live site under main/.
• concurrency group cancels older in-flight builds for the same branch, so the push and pull_request triggers don't double up.

README.md is new (none existed) and documents the branch model, local preview, the preview URL pattern, and the reminder to keep pip install lines in sync across ci.yml and preview.yml when adding plugins.

Files changed

• .github/workflows/preview.yml (new)
• README.md (new)

Test plan

• Merge PR or push another commit to binf-100-pr-preview-deploys; confirm the preview workflow runs and publishes to https://lad-lab.github.io/previews/binf-100-pr-preview-deploys/ within ~3 min.
• Open this PR; confirm a sticky comment with the preview URL appears.
• Push a follow-up commit; confirm the same comment updates (no new comment).
• Confirm https://lad-lab.github.io (live site) renders unchanged.
• Confirm mermaid block, material nav, and git-revision plugin date all render in the preview.
• On PR close (after merge or manual close), confirm previews/binf-100-pr-preview-deploys/ is removed from main and the sticky comment shows "removed".

Closes BINF-100

[github-actions]

Preview for binf-100-pr-preview-deploys removed.