Docs: a page each, and API references for ipx and the directory laid out like docs.rs (#223)

site/gen.mjs writes them from site/api.mjs, which holds each endpoint and type as the handlers
have them, and site/docs.mjs, the guides. A misspelt type in api.mjs fails the build rather than
leaving a name that links nowhere.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

site/index.html

@@ -14,7 +14,7 @@
 <header class="bar">
   <a class="home" href="/"><img src="/logo.svg" alt="" width="36" height="36">iPX</a>
   <nav aria-label="Elsewhere">
-    <a href="/docs">Docs</a>
+    <a href="/docs/">Docs</a>
     <a href="https://directory.ipxreader.app">Directory</a>
     <a class="src" href="https://git.sdf1.net/rays/ipx" title="Source code on git.sdf1.net"><img src="/gitea.svg" alt="Source code" width="28" height="28"></a>
   </nav>