[MNT] stop CI job that adds documentation build link to PR description (#608)

this PR removes the trigger for the CI job that adds a documentation
link to the PR description.

Reasons:

* the link is already present when clicking on the documentatino job
* the PR description is now used as a commit message when merged to
`main`

Author: fkiraly

.github/workflows/documentation_links.yml

@@ -1,8 +1,6 @@
 name: Read the Docs Pull Request Preview
 on:
-    pull_request_target:
-        types:
-            - opened
+    workflow_dispatch:
 
 permissions:
     pull-requests: write