[Docs] Update documentation for building Legacy C App
[Docs] Update documentation for building Legacy C App

- Reflect the recent changes in ifgen design and usage
- Using GPIO as example, describe the use case when the service
instance name is different from the API name.

Resolves: LE-14782
Change-Id: Ic68d9d68202b37a360e7c8e6c982d1b5e2c65e98

legatoproject/legato-af,legatoproject/legato-af,legatoproject/legato-af,legatoproject/legato-af,legatoproject/legato-af,legatoproject/legato-af,legatoproject/legato-af,legatoproject/legato-af,legatoproject/legato-af