Claude's suggestions for contributing docs

[penelopeysm]

Following #687, I asked Claude what it thought was still lacking on the contributing docs page, and it said:

A few things that stand out to me as gaps, roughly in order of how much they'd help newcomers:

1. Dev environment setup - There's nothing on how to actually get started developing. Cloning a repo,
   Pkg.dev-ing a local copy, running it locally. For someone who's never contributed to a Julia package
   before, this is the first wall they'd hit.
2. Codebase / ecosystem overview - The Turing ecosystem is split across many packages (Turing.jl,
   DynamicPPL.jl, AbstractMCMC.jl, Bijectors.jl, etc.). A page explaining what lives where and how they
   relate would save contributors a lot of orientation time. The Developers section covers internals, but
   there's no high-level map.
3. How to add a docs page - The Documentation page explains where docs live, but not the practical
   steps: how to add a new Quarto page, preview locally, add it to the sidebar, etc. Given that docs
   contributions are probably the most accessible entry point, this seems valuable.
4. Good first issues - The Getting Involved page links to the issue tracker but doesn't mention whether
   there's a good-first-issue label or similar. Pointing people to a filtered issue list would lower the
   barrier a lot.

which I broadly agree with. (I mean, (2) is pretty much something I suggested ages ago #606)

[penelopeysm]

I mostly solved this in #687, with the exception of good first issues, for which we don't really have a good label. At some point we tried it, but nowadays I'm not genuinely sure what is easy to fix any more, as most things in DPPL require quite in-depth knowledge. It's something I'll continue to ponder, but not something to address immediately on the docs repo.