exec_program: Add example how to upgrade to execute_process()

This hopefully helps a bit when upgrading CMake code that uses
`exec_program()` command.

Changes:
- Used third-person phrasing for the command description.
- The `exec_program()` command arguments in signature updated a bit.
- Mention of CMakeLists.txt file removed as this command also worked in
  CMake scripts and other modes.
- Added examples section.