docs(api): add releases timeline page

## Changes
- Add a Releases tab to the top navigation via a custom Starlight
  Header override (HeaderLinks.astro)
- Add ReleaseCard.astro — Vercel/Linear-style entry with sticky date
  + version on the left, headline and feature bullets on the right
- Curate 14 release entries spanning v1.0 (Sept 2024) through v3.7.1
  (April 2026), each focused on the most important user-facing
  features rather than every commit
- Newest release carries a "Latest" badge in the brand color
- Mobile layout collapses the two-column timeline to a single column

## Benefits
- Visitors can see at a glance what shipped recently and what
  Checkmate has matured into over time
- The data file (src/data/releases.ts) is the only thing that needs
  editing when a new version ships — no template or styling churn
- Built on the same Starlight site as the API docs, so look and feel
  stay consistent

docs/api/astro.config.mjs

@@ -33,6 +33,11 @@ export default defineConfig({
         github: "https://github.com/bluewave-labs/Checkmate",
       },
 
+      // Inject a "Releases" link into the site header.
+      components: {
+        SocialIcons: "./src/components/HeaderLinks.astro",
+      },
+
       sidebar: [
         {
           label: "Getting started",