docs: showcase let syntax in for_loop

[purung]

I thought it would be a boon for DX to showcase the let syntax for the <For> component.

I knew there were some improvements, but I erroneously tried the legacy let:item style to destructure my struct and had to go looking for docs. It was kind of hard to find so I thought I'd try to contribute 😅 Please, edit and adjust to your liking.

I think this should close #3059

See cousin PR for the book.

[gbj]

Thanks for the PR! I think this is a good idea. 3 of the 4 CI failures here are unrelated (semver, autofix, and any_spawner), but the leptos one is caused by the PR and needs a fix: specifically, the doctests don't compile because they're missing imports and the view macro.

It's good to have these and they double as regression tests. You should just need to add the prelude import and also a view! { wrapping the two examples and it should be fine. (You can comment these out with a # at the beginning of the line so they don't show up in the docs.) I'd recommend running cargo test locally (in the leptos crate) to make sure you've got it right.

[purung]

Thank you for the guidance! Tests pass locally now. I chose to keep whole snippets of view! {}. Let me know if you'd like me to reconsider the crop.

[gbj]

Thank you!