fix(docs): serve docs site at root of Vercel domain

Astro's base option was set to /docs to anticipate the reverse-proxy
URL at checkmate.so/docs. Vercel serves the dist folder at the root,
which made every page 404 because Astro had baked /docs/ into every
internal link and asset URL.

## Changes
- Remove base: "/docs" from astro.config.mjs — site now serves at root
- Update Releases nav link to /releases
- Update README's reverse-proxy snippet to strip the /docs prefix
  before forwarding to Vercel

## Final URL behaviour
- Direct: https://<vercel-domain>/                  → Introduction
- Direct: https://<vercel-domain>/releases/         → Releases
- Proxy:  https://checkmate.so/docs/                → strip /docs → Introduction
- Proxy:  https://checkmate.so/docs/releases/       → strip /docs → Releases

docs/api/astro.config.mjs

@@ -10,9 +10,10 @@ import starlightOpenAPI, { openAPISidebarGroups } from "starlight-openapi";
 const OPENAPI_PATH = "./src/vendored/openapi.json";
 
 export default defineConfig({
-  // Served at https://checkmate.so/docs — the leading slash is included so
-  // every internal link resolves correctly when fronted by a reverse proxy.
-  base: "/docs",
+  // The deployed site serves at the root of its Vercel domain. The
+  // reverse proxy that maps checkmate.so/docs/* → Vercel must strip
+  // the /docs/ prefix before forwarding (the Cloudflare Worker snippet
+  // in docs/api-content/README.md does this).
   site: "https://checkmate.so",
 
   integrations: [

docs/api/src/components/HeaderLinks.astro

@@ -4,7 +4,7 @@
 import Default from "@astrojs/starlight/components/SocialIcons.astro";
 ---
 
-<a class="header-link" href="/docs/releases">Releases</a>
+<a class="header-link" href="/releases">Releases</a>
 <Default><slot /></Default>
 
 <style>