Match Ergonomics 2024: add context and examples to the unstable book

[dianne]

The examples here are pretty limited and don't illustrate the differences between the two feature gates, but my hope is that they get the general idea across. I can try and add some more nuance or more comprehensive examples too if that would help.

Hopefully the doctest isn't too sneaky. I wanted to make the bindings' types explicit, and the most readable way I could think of was to use a helper.

Unfortunately it looks like the "run this code" button doesn't work yet, but I made sure the examples are cross-edition, so that should resolve on its own once playground's nightly updates (or if playground's default becomes edition 2024, or if the edition in the markdown gets forwarded to playground). It looks like the default edition on playground is now 2024, so the run button works! There's no output, but having a button to show that it compiles is nice, I think.

Relevant tracking issue: #123076

r? @Nadrieril

[Nadrieril]

Looks good, that's already a lot more helpful for someone stumbling upon these feature gates. Thanks!

@bors r+ rollup

[bors]

📌 Commit 812b1de has been approved by Nadrieril

It is now in the queue for this repository.