Broad checklist of docs bits to work on

The documentation, in particular the one in Turing.jl itself, is in dire need of update given the amount of features and improvements we've made over the past year. In particular, the tutorials have lots and lots room for improvement.

A few things that come to mind immediately are the foollowing.

User-facing side:

• Turing.predict for predicting based on a given chain.
• DynamicPPL.generated_quantities, similar to Stan's generated-block, which allows you to, effectively, capture the return-values of the model (i.e. the stuff in return ...) conditioned on a chain.
• condition and decondition. There are now two ways to indicate whether a variable is to be considered an observation: passing the variable as an argument (the "old" way), or using condition / | (the "new" way). The latter has is, arguably, more intuitive, in addition to being much easier to work with programmatically.
• @submodel. A macro that allows you to use models within models. Makes it very easy to write modular models.
• logprior, loglikelihood, and logjoint. Easy-to-use methods for evaluating the model in different ways.
• fix and condition

Developer-side:

• Implementation of the LogDensityProblems.jl interface for a @model.
• DynamicPPL.TestUtils. This is a sub-module of DynamicPPL that can be quite useful if one is developing features for Turing.

We will add more to the list as we go on, but for now this is a good starting point.

Review the Turing.jl documentation and tutorials, then use the unchecked condition/decondition, LogDensityProblems.jl interface, and DynamicPPL.TestUtils entries as the initial scope. Done means documenting those listed user-facing and developer-facing features and improving the tutorials, but the issue does not name specific files or tests.