ci: deploy surge preview for every docs PR (#92)
Adds .github/workflows/pr-preview.yaml. On every PR opened against main (and on workflow_dispatch), the workflow: - builds the mkdocs site with --strict - deploys it to https://cf-cna-pr-<N>.surge.sh - upserts a single hidden-marker comment on the PR with the URL, so subsequent pushes update the same comment instead of spamming Subdomain alias cf-cna keeps the host label under the 63-char DNS limit; the full org+repo prefix would overrun once PR numbers grow. Concurrency group cancels stale runs. Workflow fails loudly with an actionable message if SURGE_TOKEN is missing. Manual one-time setup is documented on the issue: generate a surge token via 'surge token' and set it as the SURGE_TOKEN repo secret.