Add 6.x to workflows (#205)

Author: recrwplay

.github/workflows/docs-pr-checks.yml

@@ -4,6 +4,7 @@ name: "Verify docs PR"
 on:
   pull_request:
     branches:
+      - 6.x
       - 5.x
       - 4.x
       - 3.x

.github/workflows/docs-teardown.yml

@@ -4,6 +4,7 @@ name: "Documentation Teardown"
 on:
   pull_request_target:
     branches:
+      - 6.x
       - 5.x
       - 4.x
       - 3.x