chore(showcase): Add Prevue to Site Showcase (#13370)

<!--
  Have any questions? Check out the contributing docs at https://gatsby.dev/contribute, or
  ask in this Pull Request and a Gatsby maintainer will be happy to help :)
-->

## Description

<!-- Write a brief description of the changes introduced by this PR -->

## Related Issues

<!--
  Link to the issue that is fixed by this PR (if there is one)
  e.g. Fixes #1234, Addresses #1234, Related to #1234, etc.
-->

docs/sites.yml

@@ -5381,3 +5381,12 @@
   categories:
     - Beauty
     - Business
+- title: Prevue
+  main_url: https://www.prevue.io
+  url: https://www.prevue.io
+  featured: false
+  description: >
+    All in One Prototyping Tool For Vue Developers
+  categories:
+    - Open Source
+    - Web Development