apple signing documentation

[MonicaG]

Added documentation for apple signing. This info came from:

• Apple App Signing
• Mobile Starter Kit - Code Signing

Some changes from the above documents:

• The code signing service was retired. It is no longer in use
• We are allowing Xcode to manage the developer certificates
• We are asking teams to use a pipeline to build and sign their apps vs us sending the certificates to the teams

[MonicaG]

@kaitlyn-rosenburg - Applied the changes you suggested. You can review them here. The suggestion to move the sentence is in this review.

Sorry for the multiple reviews on this. I missed the 'move sentence' request in the first commit. I applied it after.