Add automated ATD rule generation guide for Dune

[octoio]

PR checklist

• [ ] New code has tests to catch future regressions
• Documentation is up-to-date
• [ ] CHANGES.md is up-to-date

I've been using the atd project a lot in my ocaml projects. I was tired of manually updating my dune files manually and decided to take a stab at generating the dune rule for all my atd files. You can see how I personally use the generation in my project.

Hopefully this can help people 😃

[Khady]

generally looking good, thanks. We have something similar internally, really useful. Could potentially be good to ship such an executable directly.

I wonder if the mode fallback is good or not. Might be a trap for people who are not very careful. But no strong opinion yet.

[octoio]

I wonder if the mode fallback is good or not. Might be a trap for people who are not very careful. But no strong opinion yet.

I can definitely update to remove the fallback mode.