Add local Docker deployment and submodules

- docker-compose.yml: builder, nginx web server and a LaTeX container (profile "latex")
  for regenerating the SVG drawings
- Makefile, scripts/fetch.sh, .env.example
- 50ohm, 50ohm-contents-dl and 50ohm-ansible as submodules tracking branch sv

Co-Authored-By: Claude Sonnet 5.5 <[email protected]>

docker/web/default.conf

@@ -0,0 +1,13 @@
+server {
+    listen 80;
+    server_name _;
+    root /usr/share/nginx/html;
+    index index.html;
+
+    # Local deploy: never cache, so a rebuild is visible on reload.
+    add_header Cache-Control "no-store" always;
+
+    location / {
+        try_files $uri $uri/ =404;
+    }
+}