link to the API documentation
minor #4640 [Book] link to the API documentation (xabbuh)

This PR was submitted for the master branch but it was merged into the 2.3 branch instead (closes #4640).

Discussion
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[Book] link to the API documentation

| Q             | A
| ------------- | ---
| Doc fix?      | no
| New docs?     | no
| Applies to    | all
| Fixed tickets |

Commits
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fae8c40 link to the API documentation