Only run linkcheck against docs dir on PR (#4741)

# Only run linkchecker on direct changes to docs

This is a stop-gap that will speed up PRs.

Some broken links can slip through if they're embedded in doc-strings
inside the codebase.

But we'll still be running the linkchecker on master.

.github/workflows/linkcheck.yml

@@ -4,6 +4,8 @@ on:
   push:
     branches: [master]
   pull_request:
+    paths:
+      - 'docs/**'
 
 env:
   POETRY_VERSION: "1.4.2"