Updated macOS codesigning docs page

What does this MR do and why?

This MR updates the some of the contents on the codesigning page in macOS docs section. This change removes some duplication and adds more details to the instructions by pointing the reader to the consolidated instructions on the Mobile DevOps docs page here. Note: the updated docs page is still in an MR, but should be merged soon Updated Mobile DevOps docs page (!118806 - merged)

@gabrielengel_gl @DarrenEastman does this change make sense to both of you?

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