Write AF-XDP user guide

### Problem:

There is no documentation / user guide for AF-XDP. It's difficult to enable and pretty dependent on environment, so without a good user guide customers are unlikely to be able to successfully use it.

### Solution:

Document how to enable AF-XDP.

* **Does this change what s2n-quic sends over the wire?** No
* **Does this change any public APIs?** --> No

### Requirements / Acceptance Criteria:

A customer with minimal understanding of quic / xdp should be able to enable AF-XDP by following the user guide.

* **RFC links:** Links to relevant RFC(s)
* **Related Issues:** Link any relevant issues
* **Will the Usage Guide or other documentation need to be updated?** Yes
* **Testing:** Let someone other than camshaft or Wesley follow the guide and check if they succeed :)

### Out of scope:

[//]: # (NOTE: If you believe this might be a security issue, please email aws-security@amazon.com instead of creating a GitHub issue. For more details, see the AWS Vulnerability Reporting Guide: https://aws.amazon.com/security/vulnerability-reporting/ )

The issue names no documentation files, tests, or entry points. Start by locating the existing usage guide and AF-XDP enablement code or configuration, then document the environment-dependent steps, relevant RFCs, and related issues. Done means a reviewer unfamiliar with QUIC and XDP can follow the guide successfully, with an independent user validating it.

Written by the indexing model from the issue text.