Add warning regarding `pull_request_target` event

[ylemkimon]

What article on docs.github.com is affected?

https://docs.github.com/en/free-pro-team@latest/actions/reference/events-that-trigger-workflows

What part(s) of the article would you like to see updated?

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.

[welcome]

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