Add help-osk-docs stack: Zensical instructions site for 05-osk

Static instructions site for client 05 served at help.osk.team. Markdown +
images built by Zensical (zensical build) into public/, served by nginx behind
Traefik. Content is bind-mounted from /mnt/containers/help-osk-docs (absolute
host path — Portainer GitOps can't resolve relative mounts). Adds the
help/help-osk router+service to traefik/dynamic/web.yml (TLS via Let's Encrypt,
public/open).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

help-osk-docs/nginx.conf

@@ -0,0 +1,31 @@
+# Serves the static site built by Zensical (./public) for client 05.
+# Mounted at /etc/nginx/conf.d/default.conf inside nginx:alpine.
+
+server {
+    listen 80;
+    server_name _;
+
+    root  /usr/share/nginx/html;
+    index index.html;
+
+    # Zensical uses directory-style URLs (/page/ -> /page/index.html).
+    # Unknown paths fall through to the generated 404 page.
+    error_page 404 /404.html;
+    location / {
+        try_files $uri $uri/ =404;
+    }
+
+    # Long-cache fingerprinted theme assets; keep HTML uncached so content
+    # updates show up immediately after a rebuild.
+    location /assets/ {
+        expires 7d;
+        add_header Cache-Control "public";
+    }
+    location = /404.html {
+        internal;
+    }
+
+    gzip            on;
+    gzip_types      text/css application/javascript application/json image/svg+xml;
+    gzip_min_length 1024;
+}