Only run docs workflow on master branch pushes

This shouldn't be run for PRs.

.github/workflows/docs.yml

@@ -4,13 +4,6 @@ on:
   push:
     branches:
       - master
-    tags:
-      # Run when pushing version tags, since otherwise it's impossible to
-      # restart a successful build after pushing a tag
-      - '*.*.*'
-  pull_request:
-    branches:
-      - master
 
 defaults:
   run: