gha: Add workflow_dispatch trigger to docs-url-alive-check

We can't test this PR because the workflow needs this trigger, so adding
this will allow testing future PRs.

Signed-off-by: Aurélien Bombo <abombo@microsoft.com>

.github/workflows/docs-url-alive-check.yaml

@@ -1,6 +1,7 @@
 on:
   schedule:
     - cron:  '0 23 * * 0'
+  workflow_dispatch:
 
 permissions: {}