feat(docs): add Nexus Shield guardrail integration guide

[baturhantasdelen-sudo]

Summary

Adds integration documentation for Nexus Shield — a sub-10ms in-RAM PII sanitization and security guardrail proxy for Portkey routing.

Changes Included

• Added integration guide with code examples showing how to pass base_url and custom headers for Nexus Shield with Portkey.

Usage Preview

from portkey_ai import Portkey

portkey = Portkey(
    api_key="PORTKEY_API_KEY",
    base_url="[https://api.nexusshield.ai/v1](https://api.nexusshield.ai/v1)",
    custom_headers={"x-nexus-api-key": "nx_live_YOUR_KEY"}
)

response = portkey.chat.completions.create(
    model="gpt-4o",
    messages=[{"role": "user", "content": "Hello, my phone is 555-0199"}]
)

print(response)
Checklist
[x] Verified Markdown formatting

[x] Followed repository contribution guidelines