docs(iroh): add documentations and examples for the `iroh::node::Clie…

…nt` (#2582)

## Description

Happy Docs Day! Added documentation to the `iroh::node::Client`,
including examples.

## Change checklist

- [x] Self-review.
- [x] Documentation updates following the [style
guide](https://rust-lang.github.io/rfcs/1574-more-api-documentation-conventions.html#appendix-a-full-conventions-text),
if relevant.

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Co-authored-by: Kasey Huizinga <ramfox@Kaseys-MBP.lan>