feat: allow documentation url to be changed

[shreddedbacon]

The remote-controller has the ability to inject any variables defined against it that start with LAGOON_FEATURE_FLAG_ into build pods.

This sets the default documentation url to https://docs.lagoon.sh but allows for the remote-controller to override this
Adding LAGOON_FEATURE_FLAG_DEFAULT_DOCUMENTATION_URL with the url of the alternative documentation, this way references to documentation can point to a version suited for custom installations, as long as the alternative documentation paths are the same.

[tobybellwood]

Yup - this is perfect