docs(docs): fix 404 on v3 config-migration page from v4 upgrade guide (#2429)

The link from /v4-0-0-snapshot/upgrading/2x-to-3x/ to the v3 Config
Directory Migration page 404'd because the file was named
3.0.0-config-migration.md and Astro's slug generator strips dots
entirely rather than converting them to dashes, producing the
unintuitive URL /v3-0-0/upgrading/300-config-migration/. PR #2181
introduced the link assuming dots-to-dashes — that assumption was
incorrect.

- Rename file to 3-0-0-config-migration.md so the URL matches every
  other doc filename in the tree and the existing link works as written.
- Add redirect from /300-config-migration/ to preserve any external
  bookmarks accumulated while the dotted-slug URL was live.
- Add a sidebar entry under INTRODUCTION > Upgrading; the page was
  orphaned from v3-0-0.json entirely.

Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: bpamiri

web/sites/guides/astro.config.mjs

@@ -78,6 +78,12 @@ export default defineConfig({
 		// documented Phase 0 URLs are redirected.
 		'/v4-0-0-snapshot/cli-reference': '/v4-0-0-snapshot/command-line-tools/',
 		'/v4-0-0-snapshot/cli-reference/info': '/v4-0-0-snapshot/command-line-tools/',
+		// Astro/Starlight slugs the dotted filename `3.0.0-config-migration.md`
+		// down to `300-config-migration` (github-slugger strips dots). The file
+		// was renamed to `3-0-0-config-migration.md` so the URL matches every
+		// other doc in the tree; this redirect preserves the original ugly slug
+		// for any external bookmarks accumulated while it was live.
+		'/v3-0-0/upgrading/300-config-migration': '/v3-0-0/upgrading/3-0-0-config-migration/',
 	},
 	integrations: [
 		starlight({

…-0-0/upgrading/3.0.0-config-migration.md …-0-0/upgrading/3-0-0-config-migration.mdweb/sites/guides/src/content/docs/v3-0-0/upgrading/3.0.0-config-migration.md renamed to web/sites/guides/src/content/docs/v3-0-0/upgrading/3-0-0-config-migration.md web/sites/guides/src/content/docs/v3-0-0/upgrading/3.0.0-config-migration.md renamed to web/sites/guides/src/content/docs/v3-0-0/upgrading/3-0-0-config-migration.md

@@ -42,7 +42,13 @@
       },
       {
         "label": "Upgrading",
-        "link": "/v3-0-0/introduction/upgrading/"
+        "link": "/v3-0-0/introduction/upgrading/",
+        "items": [
+          {
+            "label": "Config Directory Migration (3.0.0)",
+            "link": "/v3-0-0/upgrading/3-0-0-config-migration/"
+          }
+        ]
       },
       {
         "label": "Screencasts",