docs: scrape example code from examples/*

[joshka]

see https://doc.rust-lang.org/nightly/rustdoc/scraped-examples.html

I noticed this in another library and thought it was a pretty neat idea. We have a bunch of examples that we can use for this.

Here's how it shows up:

To test this locally, use:

rustup toolchain add nightly
cargo +nightly doc -Zunstable-options -Zrustdoc-scrape-examples --all-features --open

[orhun]

Wow, pretty neat feature 🦀