docs: Add find_program_address and example

[joncinque]

Problem

There were some questions about using create_program_address, and typically find_program_address is a part of that development flow, but the docs don't have any mention or examples using find_program_address.

Summary of Changes

Add the find_program_address implementation and a similar escrow example using find_program_address instead of create_program_address.

Fixes #

[jackcmay]

Looks great, thanks!