docs: add universal-cache to third-party middleware docs

[lord007tn]

Summary

This docs PR accompanies @hono/universal-cache from:

• feat(universal-cache): add universal cache middleware package middleware#1764
• discussion origin: Universal Cache Middleware with Storage Adapter Support hono#3857

The package depends on unstorage, so cache backends are adapter-based while keeping a single API surface in Hono.

What this PR adds

• new third-party middleware page for @hono/universal-cache
• sidebar entry for the new page
• links from the third-party middleware index
• documented default behavior and defaults
• documented opt-in manual revalidation with shouldRevalidate
• documented workerd stale refresh behavior

Validation

• npx prettier --check .vitepress/config.ts docs/middleware/third-party.md docs/middleware/third-party/universal-cache.md

Merge Note

Please merge this together with the middleware PR:

• feat(universal-cache): add universal cache middleware package middleware#1764