Adds a GitHub Action to notify when preview link is live

[AlannaBurke]

Description

Adds a GitHub Action to notify when preview link is live. Waits up to 40 minutes. I did test this locally to the extent that i could.

Checklist

• The issue that is being fixed is referred in the description (see above "Fixes #ISSUE_NUMBER")
• Only one issue is addressed in this pull request
• Labels from the issue that this PR is fixing are added to this pull request
• No unnecessary issues are included into this pull request.

[pytorch-bot]

🔗 Helpful Links

🧪 See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/tutorials/3702

• 📄 Preview Python docs built from this PR

Note: Links to docs will display an error until the docs builds have been completed.

✅ No Failures

As of commit c5f9570 with merge base a3695df ():
💚 Looks good so far! There are no failures yet. 💚

This comment was automatically generated by Dr. CI and updates every 15 minutes.

[AlannaBurke]

Looks like there is an unrelated error, i took a stab at fixing it here but i haven't gotten it to build yet: #3704

[svekars]

we don't need this.

[github-actions]

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