Add tutorial on enumerative inference

[ztangent]

I had meant to add a tutorial on how to use enumerative inference to debug modeling errors after adding enumerative inference functionality in #545. This PR adds the tutorial, which covers:

• Exact enumeration over discrete models
• Approximate enumeration over continuous models
• Diagnosing model misspecification (vs. inference failure)
• Addressing model misspecification

In addition, this PR ports over the MCMC tutorial from the https://gen.dev website to the documentation itself (a task which had been only partially completed), and fixes other minor issues in the documentation. The enumerative inference tutorial builds upon the MCMC tutorial, showing how enumeration can be applied to a linear regression model with outliers.

As noted in #545, this tutorial should help to provide a simpler entry point to beginner users of Gen, who may instead turn to importance sampling to debug their models.

Once this PR is merged, we should be able to release the 0.4.8 documentation of Gen.jl, which will also ensure the documentation reorg from #543 gets pushed to the stable branch.

The documentation preview for this PR is available here: https://www.gen.dev/docs/previews/PR554