GetPrerequisites: Sync documentation

* GetPrerequisites:
  - Added intro code block showing how to include this module.
  - Renamed "functions" to "commands".
  - Moved command descriptions above their signatures.
  - Added initial example of migrating the `gp_append_unique()` command.

* BundleUtilities:
  - Replaced obsolete gp_append_unique(): Instead of using the
    deprecated `gp_append_unique()`, these usages can be replaced with
    `if()` and `list(APPEND)` commands directly.