added that build forked pull requests is not currently supported on the GitHub Apps integration

Description

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Reasons

A link to a GitHub and/or JIRA issue (if applicable). Otherwise, a brief sentence about why you made these changes.

Please follow our style when contributing to CircleCI docs. Our style guide is here: https://circleci.com/docs/style/style-guide-overview.

Please take a moment to check through the following items when submitting your PR (this is just a guide so will not be relevant for all PRs) 😸:

[ ] Break up walls of text by adding paragraph breaks.
[ ] Consider if the content could benefit from more structure, such as lists or tables, to make it easier to consume.
[ ] Keep the title between 20 and 70 characters.
[ ] Consider whether the content would benefit from more subsections (h2-h6 headings) to make it easier to consume.
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