Docs: Update doctest info, enhance stdlib script (#58503)
- Revise `doc/README.md` to mention `JULIA_EXECUTABLE` usage with
doctests, adding a warning regarding built-in stdlibs
- Enhance `contrib/print_sorted_stdlibs.jl` script to add
`--only-sysimg` option to accurately list standard libraries built into
the system image.
These modifications aim to provide clearer documentation and more
effective tooling for contributors and AI agents wanting to run the
doctests without building julia from scratch.
Co-authored-by: google-labs-jules[bot] <161369871+google-labs-jules[bot]@users.noreply.github.com>
