FindSquish: Update documentation

- Updated and synced module documentation according to other similar
  find modules.
- Added a separate examples section.
- Commands are listed using the `command` directives.
- Updated documentation for the `SquishTestScript.cmake` file. This is
  not actually a module but a test script that `FindSquish` uses and
  also isn't technically deprecated as it is still used. Documentation
  changed a bit to explain it is an internal script that shouldn't be
  used in the project directly.