refactor: remove showcase store & registry; update .gitignore

Author: mertsincan

.gitignore

@@ -50,8 +50,8 @@ yarn-error.log*
 *.tsbuildinfo
 next-env.d.ts
 
-# contentlayer
-.contentlayer
-
-# registry
-registry.json
+# showcase
+apps/showcase/.contentlayer
+apps/showcase/registry.json
+apps/showcase/__store__/
+apps/showcase/public/r/