Transform writer's guide

Having written a few "new-style" transforms between a few people, we might compile a guide:

- intervention points for transforms in the thunder.jit flow,
- expected trace properties and how they might break and how to preserve them (and checking them in debug mode #1180 ),
- tooling
- lower level / editing traces directly
- handling proxies (new, replace, ...)
- finding things (produces and consumers)
- scopes
- `sym.bind(...)` vs. `sym(...)` vs. `bsym.from_bsym` and when to use what,
- bsym.from_bsym_swap_proxies
- adding inputs and outputs,
- higher level
- interpret_trace,
- visitor_transform,

(feel free to add)

cc @borda @apaz-cli @riccardofelluga

Start with the thunder.jit flow and the transform tooling named in the issue, including interpret_trace and visitor_transform. The guide should cover intervention points, trace properties and debug checking, proxy and scope handling, symbol APIs, adding inputs and outputs, and related lower- and higher-level workflows.

Written by the indexing model from the issue text.