docs: add section about installing ts-jest

[nate-io]

Adding the command to install ts-jest since it is an implied install here and may lead to the end user only running the @types/jest command which will not allow the tests to run.

Summary

Test plan

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[SimenB]

thanks! could you add it to all versioned docs as well?

[nate-io]

as requested @SimenB - let me know if you need anything else!

[SimenB]

thanks!

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