Variable transformations

[penelopeysm]

This PR adds a series of new docs on:

• why variables might need to be transformed;
• how this is accounted for mathematically with the Jacobian term;
• basic API and functionality of Bijectors.jl;
• higher-level usage of transforms in DynamicPPL and Turing.

Comments are welcome:)

PS. I know that there is some subtlety with the Turing.jl HMC sampling section because the print statements don't actually happen during the sampling, they occur when values_as_in_model is called (post-sampling) to collect the values. I figured it's ok to gloss over that, though, because it's still true that HMC sampling occurs entirely in unconstrained space.

[github-actions]

Preview the changes: https://turinglang.org/docs/pr-previews/582
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[willtebbutt]

Just minor stuff. I like these docs a lot!

[mhauru]

This is great! I had a few small comments, but they are minor enough that I'm happy to merge as-is if need be.

[penelopeysm]

I'll leave this up a few more days to see if anybody has more input before merging:)