[Vertex AI] Make GenerationConfig properties internal

[andrewheard]

Made the GenerationConfig properties internal to align with the Android SDK (access levels were already aligned in all other files). Updated the documentation and moved it into the parameters since the properties are no longer public (DocC links will not work). This PDF of the generated DocC may be easier to read.

[andrewheard]

cc: @rachelsaunders We may want to add some of the new configuration options to https://firebase.google.com/docs/vertex-ai/model-parameters eventually and link there instead. It could be difficult to keep details about supported model versions in sync with the Cloud docs for GenerationConfig though so we may want to link there regardless.

[rachelsaunders]

cc: @rachelsaunders We may want to add some of the new configuration options to https://firebase.google.com/docs/vertex-ai/model-parameters eventually and link there instead. It could be difficult to keep details about supported model versions in sync with the Cloud docs for GenerationConfig though so we may want to link there regardless.

This is content that we're looking at sharing directly between the docsets in the backend. For now, it's fine to link over to the Cloud docs till that happens :-)