Document how to use `INIT_` values for customizing init experience

[mjnagel]

UDS CLI includes a number of special INIT_ values that can be used to customize the init experience for a bundle. These values can configure the registry, git experience, etc and may be necessary in some environments.

Definition of done: New how-to guide on the docs website that covers how to provide these custom init values when deploying a bundle.

Note: This is possible using special UDS_ envs or the uds-config file. Examples should be provided for both.