Rollup merge of #154795 - ZuseZ4:autodiff-general-docs, r=oli-obk

Add more info about where autodiff can be applied

It's taken quite a few years, but we finally have a PR open to distribute Enzyme: rust-lang/rust#154754
I therefore went over the docs once more and noticed we don't explain a lot of the most basic features, which we added over the years and have since taken for granted.

@Sa4dUs, do you think there are more interesting cases that we are missing?

Generally, there's still a lot of complexity in it, especially for people who haven't used Enzyme before.
To some extent, that's just a result of my general design goal to expose all performance-relevant features of Enzyme, and let users explore nice abstractions on top if it, via crates. Since we don't have those nightly users yet, users haven't had time to build nicer abstractions on top of it.

I also feel like a more guided book would be a better first introduction to Enzyme, but for now I just focused on the list of features.

r? @oli-obk

Author: JonathanBrouwer