Ensure all model servers have pinned requirements and the full requirements.txt is included in the docs

[axsaucedo]

Currently the requirements on the sklearn model server are not pinned down, resulting in un-predictable "latest" versions for the dependencies available. We'll need to pin down these versions and ensure that all the dependencies are explicitly provided in the documentation. It may be worth adding a mention in the prepackaged server pages linking to the custom server doc page to specify that the image can be overriden with custom images with specific dependencies as required. Relevant to #2012.

[axsaucedo]

There is currently also not explicit description that joblib instead of sklearn.extensions.joblib should be used. We shoudl specify and provide an example of how the model server expects the models to be exported.