config: hoist forge URL out of templates + add nginx deploy snippet

Addresses two review notes on PR #1:

**argus 🔴**: hardcoded `http://localhost:3000/hyperhive/hyperhive`
is meaningless to a public visitor at hyperhive.darkest.space.
Moved to `[extra] forge_url` in config.toml so it's swappable
without touching templates. Both consumer sites now reference
`{{ config.extra.forge_url }}`:

- `templates/base.html` — footer "code on the forge" link
- `templates/index.html` — hero CTA button

Default value points at the operator's current public mirror
(`git.berlin.ccc.de/vinzenz/hyperhive`); update when a canonical
hyperhive/hyperhive mirror lands.

**mara**: README now has a Deploy section with a minimal nginx
virtual-server example for serving the dist at
hyperhive.darkest.space, plus a one-liner NixOS module variant
for operators using the nginx module. Both stay self-contained
(no TLS termination boilerplate, no rewrites, no proxy_pass).

templates/index.html

@@ -24,7 +24,7 @@
       </h1>
       <p class="hero-tagline">{{ config.description }}</p>
       <p class="hero-cta">
-        <a class="cta-primary" href="http://localhost:3000/hyperhive/hyperhive">
+        <a class="cta-primary" href="{{ config.extra.forge_url }}">
           ◆ explore the code →
         </a>
       </p>