Documentation in dire need of updates

[torfjelde]

There are some sections that are in a dire need for an update, so I figured I'd make a single issue to keep track of this:

• Performance section
  • Is memoization still a recommendation? Are there no dangers with this and AD?
  • More on vectorization of Turing models
  • Some stuff on "internals", e.g. avoiding indexing which results in "simple" and type-stable internal values.
  • Choice of AD backend
    • Needs a general update
    • Usage of TuringBenchmarks.jl should probably be encouraged
• New undocumented features
  • fix and unfix and related
  • condition and decondition (should rename this to uncondition) and related

More?

[yebai]

I fixed a few during the Quarto migration. Now that the Quarto-based workflow for docs is more friendly, we should push to fix everything.

[penelopeysm]

cf. #512