docs(sites): Added Sheffielders (#11979)

* Added Sheffielders to showcase

* Update sites.yml

* Fix linting issue

docs/sites.yml

@@ -4387,3 +4387,14 @@
   built_by: Node.js Website Redesign Working Group
   built_by_url: https://github.com/nodejs/website-redesign
   featured: false
+- title: Sheffielders
+  main_url: https://sheffielders.org
+  url: https://sheffielders.org
+  source_url: https://github.com/davemullenjnr/sheffielders
+  description: >
+    A collective of businesses, creatives, and projects based in Sheffield, UK.
+  categories:
+    - Directory
+  built_by: Dave Mullen Jnr
+  built_by_url: https://davemullenjnr.co.uk
+  featured: false