docs: add documentation for LAGOON_SERVICE_TYPES variable

[sjerdo]

Hi!

I can't really seem to find the documentation for variable LAGOON_SERVICE_TYPES

We are currently using this variable for multiple projects, to enable some services just for the production environment.
But if I search for LAGOON_SERVICE_TYPES in the documentation, I can't find any related documentation. I can only find the examples in the test, in the build-deploy-tool source code and in this related MR #2156

There was some discussion in #2156 about putting this feature in public documentation