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).

config.toml

@@ -32,3 +32,9 @@ smart_punctuation = true
 # theme intent is reviewable in one place.
 palette = "catppuccin-mocha"
 accent = "amber"
+
+# Public-facing forge URL — surfaced in templates via
+# `{{ config.extra.forge_url }}`. Update this when the canonical
+# public hyperhive mirror lands; the current value is the operator's
+# berlin.ccc.de instance (per the project README at that mirror).
+forge_url = "https://git.berlin.ccc.de/vinzenz/hyperhive"