Submodel docs

[penelopeysm]

• The two 'outputs' of a model: return value + random variables and how to extract them
• Basic submodels, prefixing
• Sharp edges with this (i.e. likelihoods in submodels - can't condition on a ~ to_submodel(...))
• How to actually condition values in submodels
• Look to the future Unification of submodels and distributions TuringLang/Turing.jl#2485 and technical work in [wip] submodel explorations TuringLang/DynamicPPL.jl#815