docs(iroh-net): Update endpoint docs (#2334)

## Description

This rewrites quite a bit of the docs aiming to be more consistent and
clearly describe the various parts of the API and how they interact.

It also tries to get the style standardised following
https://github.com/rust-lang/rfcs/blob/master/text/1574-more-api-documentation-conventions.md#appendix-a-full-conventions-text

The order of the functions has been deliberately changed, the order is
used by rust doc as well so directly affects how users see the
documentation.

Some `PublicKey` types have been changed into `NodeId`.  They mostly
already had `node_id` as parameter names and were described as such.
But no other code changes have been made.


## Breaking Changes

<!-- Optional, if there are any breaking changes document them,
including how to migrate older code. -->

## Notes & open questions

The sorting makes the diff rather difficult to read, sorry about that.
But
maybe that's not so bad for the doc comments as the result is more
important
than the diff.

I've taken to calling the logical thing the `Endpoint` controls an
"iroh-net node". This is the thing that goes into the `NodeMap` etc, so
is consistent with naming. But I'm usually using "iroh-net node" in
prose to distinguish it from the `iroh::node::Node`.

## Change checklist

- [x] Self-review.
- [x] Documentation updates if relevant.
- ~~[ ] Tests if relevant.~~
- ~~[ ] All breaking changes documented.~~

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Co-authored-by: Divma <[email protected]>