fix(iroh-cli)!: Improve cli and configuration file (#2532)

## Description

The configuration file behaviour is improved:

- Unknown fields will now cause an error rather than be silently
ignored.

- Make it possible to express GcPolicy in the TOML config file.  This
  was not possible before.  It was possible to disable it before, but
  it was disabled by default so that was rather moot.

The --help output is improved:

- Update header of iroh.
- Directly point to config file docs.
- Use consistent style.


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## Breaking Changes

- Unknown fields in the configuration file will now cause an error.

- Configuring the GC Policy in the configuration file has changed.

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including how to migrate older code. -->

## Notes & open questions

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## Change checklist

- [ ] 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.
- [x] Tests if relevant.
- [x] All breaking changes documented.