feat(ci): enable preview deployments via docs-preview label

Add support for triggering preview deployments by adding the
'docs-preview' label to pull requests. This enables on-demand testing
of documentation changes before merging.

When the label is present:
- DEPLOY_ENABLED is set to "true"
- DEPLOY_ENVIRONMENT is set to "preview"
- deploy job runs and calls cf-deploy-preview recipe
- Version is uploaded with branch-specific preview alias

This preserves the sophisticated version promotion workflow while
adding label-based deployment control, following a pattern similar
to existing skip-ci and actions-debug labels.

Preview URL format: https://b-{branch}-ts-nix-docs.sciexp.workers.dev

Author: cameronraysmith

.github/workflows/ci.yaml

@@ -109,6 +109,10 @@ jobs:
             if ${{ contains(github.event.pull_request.labels.*.name, 'actions-debug') }}; then
               DEBUG="true"
             fi
+            if ${{ contains(github.event.pull_request.labels.*.name, 'docs-preview') }}; then
+              DEPLOY_ENABLED="true"
+              DEPLOY_ENVIRONMENT="preview"
+            fi
             CHECKOUT_REF="${{ github.event.pull_request.head.ref }}"
             CHECKOUT_REV="${{ github.event.pull_request.head.sha }}"
           else