CI: Queue up doc pushes (#206)

This action fails if more than 1 is running at the same time (which
happens if you merge multiple PRs in quick succession). Fix is by
disabling concurrency, so they just queue up.

Author: klieret

.github/workflows/build-docs.yaml

@@ -8,6 +8,12 @@ on:
   pull_request:
     branches:
       - main
+
+# Prevent concurrent runs that could conflict when pushing to gh-pages
+concurrency:
+  group: build-docs-${{ github.ref }}
+  cancel-in-progress: false
+
 permissions:
   contents: write
 jobs: