feat(showcase): add Dawn Labs site (gatsbyjs#9931)

<!--
  Q. Which branch should I use for my pull request?
  A. Use `master` branch (probably).

  Q. Which branch if my change is a bug fix for Gatsby v1?
  A. In this case, you should use the `v1` branch

  Q. Which branch if I'm still not sure?
  A. Use `master` branch. Ask in the PR if you're not sure and a Gatsby maintainer will be happy to help :)

  Note: We will only accept bug fixes for Gatsby v1. New features should be added to Gatsby v2.

  Learn more about contributing: https://www.gatsbyjs.org/docs/how-to-contribute/
-->
Add Dawn Labs' site to the showcase

docs/sites.yml

@@ -3346,3 +3346,13 @@
   built_by: Cameron Rison
   built_by_url: https://github.com/killakam3084
   featured: false
+- title: Dawn Labs
+  url: https://dawnlabs.io
+  main_url: https://dawnlabs.io
+  description: >-
+    Thoughtful products for inspired teams. With a holistic approach to engineering and design, we partner with startups and enterprises to build for the digital era.
+  categories:
+    - Technology
+    - Agency
+    - Web Development
+  featured: false