Add manual trigger to RTD preview, run workflow on forks. (#1083)

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Author: nikoshell

.github/workflows/rtd-preview.yml

@@ -3,12 +3,14 @@ on:
   pull_request_target:
     types:
       - opened
+  workflow_dispatch:
 
 permissions:
   pull-requests: write
 
 jobs:
   documentation-links:
+    if: github.event.pull_request.head.repo.fork == true
     runs-on: ubuntu-latest
     timeout-minutes: 10
     steps: