Merge #1788

1788: Create a PR comment with docs preview link r=ToucheSir a=logankilpatrick

It looks like per `@ToucheSir` 's comment here: #1784, there is already a documentation preview workflow setup. This simply adds a comment to the PR with a link like we do on the JuliaLang.org website repo.

Co-authored-by: Logan Kilpatrick <23kilpatrick23@gmail.com>

Author: bors[bot]

.github/workflows/pr_comment.yml

@@ -0,0 +1,14 @@
+name: pr_comment
+on:
+  pull_request:
+    types: [opened]
+jobs:
+  pr_comment:
+    runs-on: ubuntu-latest
+    steps:
+      - name: Create PR comment
+        if: github.event_name == 'pull_request' && github.repository == github.event.pull_request.head.repo.full_name # if this is a pull request build AND the pull request is NOT made from a fork
+        uses: thollander/actions-comment-pull-request@71efef56b184328c7ef1f213577c3a90edaa4aff
+        with:
+          message: 'Once the build has completed, you can preview any updated documentation at this URL: https://fluxml.ai/Flux.jl/previews/PR${{ github.event.number }}/ in ~20 minutes'
+          GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}