chore(deps): upgrade dependencies for Astro 6 (DOCSDEV-60) (#660)

Upgrade core: astro 5.x -> 6.x, starlight 0.37 -> 0.38,
docs-design-system 0.11 -> 0.12, starlight-links-validator 0.19 -> 0.23.
Migrate i18n collection to use loader (Astro 6 requirement).
Add explicit sidebar options to starlightFullViewMode for Astro 6 compat.
Fix anchor link in get-started.mdx caught by upgraded link validator.

Author: Infi-Knight

astro.config.mjs

@@ -34,7 +34,10 @@ export default defineConfig({
           errorOnFallbackPages: false,
           exclude: ['/prob-revshare']
         }),
-        starlightFullViewMode()
+        starlightFullViewMode({
+          leftSidebarEnabled: true,
+          rightSidebarEnabled: true
+        })
       ],
       expressiveCode: {
         styleOverrides: {