Air-gap fixes + in-app /docs (README/DEPLOYMENT/api.md/docs/*)

Air-gap:
- DynamicRecord.refreshAll() called getPublicIp() (api.ipify.org,
  icanhazip.com, ifconfig.me) every 4h on a timer regardless of
  whether any dynamic records were configured -- the one background
  call in the repo not actually gated by feature use. Now skips the
  lookup entirely when there's nothing to refresh.
- Removed the stray, unauthenticated GET /test page (a leftover
  jq-repeat demo) that loaded jQuery + Mustache from external CDNs.
- Removed a dead IE<9-only html5shim script tag pointing at a domain
  that no longer resolves.

Docs:
- New GET /docs (index) and /docs/:slug routes render this project's
  own README, DEPLOYMENT, api.md, and docs/*.md server-side via
  marked (new dependency) -- so the documentation is readable from
  the running app with no route to GitHub Pages, where it otherwise
  only lives. Public, no auth, same tier as the health endpoint.
- .dockerignore/Dockerfile updated: docs/, DEPLOYMENT.md, and
  nodejs/api.md were previously excluded from the image entirely
  ("served via GitHub Pages, not from the image") -- now copied in
  alongside README.md/tos.md-style, since they're needed at runtime.

nodejs/views/bottom.ejs

@@ -10,6 +10,9 @@
 					<a href="https://github.com/theta42/proxy/blob/master/LICENSE" target="_blank" class="text-light">MIT License</a>
 				</span>
 				<span class="d-flex align-items-center gap-3">
+					<a href="/docs" class="text-light text-decoration-none">
+						<i class="fa-solid fa-book"></i> Docs
+					</a>
 					<a href="https://github.com/theta42/proxy" target="_blank" class="text-light text-decoration-none">
 						<i class="fa-brands fa-github"></i> GitHub
 					</a>