Promote authServerRef tip to a proper subsection in auth guide

[danbarr]

Problem

The authServerRef field documentation in guides-k8s/auth-k8s.mdx (introduced in #685) is wrapped in a :::tip admonition, but it's ~25 lines with a full YAML example and is the only documentation for authServerRef outside the auto-generated CRD spec.

This makes it primary feature documentation masquerading as a helpful aside. A tip implies "nice to know," but anyone needing to combine embedded auth with outgoing token exchange has no other place to find this information.

Suggested approach

• Promote the tip to a dedicated H3 subsection under the embedded auth section, e.g. "### Combine embedded auth with outgoing token exchange".
• This makes it discoverable via the page's table of contents and signals to readers that it's a real configuration path, not a bonus trick.

Context

Identified during editorial review of #685 (Part 1: Update docs for ToolHive v0.17.0).