Add warning regarding pull_request_target event

[ylemkimon]

Why:

pull_request_target event runs in the context of the base repository, which could be potentially dangerous if it is used to test untrusted code from public forks. It also shares the same scope of cache with the base branch. Closes #1353.

What's being changed:

Check off the following:

• All of the tests are passing.
• I have reviewed my changes in staging.
• For content changes, I have reviewed the localization checklist
• For content changes, I have reviewed the Content style guide for GitHub Docs.

[welcome]

Thanks for opening this pull request! A GitHub docs team member should be by to give feedback soon. In the meantime, please check out the contributing guidelines.

[janiceilene]

@ylemkimon Thanks so much for opening a PR! I'll get this triaged for review ✨

[github-actions]

This PR is stale because it has been open 7 days with no activity and will be automatically closed in 3 days. To keep this PR open, update the PR by adding a comment or pushing a commit.

[janiceilene]

Thanks for your patience @ylemkimon! Our small team is working our way through all the amazing contributions ✨

[github-actions]

This PR is stale because it has been open 7 days with no activity and will be automatically closed in 3 days. To keep this PR open, update the PR by adding a comment or pushing a commit.

[janiceilene]

@martin389 It looks like @ylemkimon incorporated your suggestions 🎉 Can you add a ready to merge label if this PR is all set?

[martin389]

Thanks @janiceilene! For the next step, I'm checking that support approves this guidance 👍

[elstudio]

Looks good -- and a very very good thing to warn folks about!

[github-actions]

Thanks very much for contributing! Your pull request has been merged 🎉 You should see your changes appear on the site in approximately 24 hours.

[martin389]

Thank you @ylemkimon! 👍

[ylemkimon]

Thank you everyone for your reviews and the great work at GitHub!