docs: Also list iroh-gossip as a re-export (#2606)

## Description

<!-- A summary of what this pull request achieves and a rough list of
changes. -->

Just a missing doc change that I noticed when trying to answer a Q on
discord.

## Breaking Changes

<!-- Optional, if there are any breaking changes document them,
including how to migrate older code. -->
None

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

Author: matheus23

iroh/src/lib.rs

@@ -80,6 +80,7 @@
 //! - [iroh_base] as [`base`]
 //! - [iroh_blobs] as [`blobs`]
 //! - [iroh_docs] as [`docs`]
+//! - [iroh_gossip] as [`gossip`]
 //! - [iroh_net] as [`net`]
 //!
 //! ## Feature Flags