docs: add developer page for GC debug build tools (#61637)

As far as I could tell we never added documentation to these debug
tools,
partly explaining why they keep bit-rooting.

Document the debugging tools enabled by building Julia with
`WITH_GC_DEBUG_ENV=1`, including the `JULIA_GC_ALLOC_POOL`,
`JULIA_GC_ALLOC_OTHER`, `JULIA_GC_ALLOC_PRINT`, and
`JULIA_GC_WAIT_FOR_DEBUGGER` environment variables, the `GC_VERIFY`
write-barrier verifier, and tips for debugging write-barrier violations
using GDB, ASLR disabling, and `rr` record-replay.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>