docs: add auth_proto switch discovery example

[dacasti2mx]

Summary

• Add docs/SWITCH_DISCOVERY_AUTH_PROTO_GUIDE.md — end-to-end example for configuring auth_proto, the SNMPv3 authentication and privacy protocol NDFC uses during normal initial switch discovery.
• Add a cross-reference note to docs/SWITCH_DISCOVERY_CREDENTIALS_GUIDE.md to distinguish POAP/preprovision flows from normal discovery.

Files changed

File	Change
docs/SWITCH_DISCOVERY_AUTH_PROTO_GUIDE.md	New file — full example
docs/SWITCH_DISCOVERY_CREDENTIALS_GUIDE.md	Add cross-reference note

What the example covers

• What auth_proto controls and the four data model paths where it applies
• Table of the six supported values with authentication and privacy algorithms
• NX-OS prerequisite: SNMPv3 user configuration commands
• Environment variable requirements (NDFC_SW_USERNAME / NDFC_SW_PASSWORD) and how they must match the NX-OS user
• End-to-end SHA_AES example (NX-OS → env vars → data model → playbook run)
• Scope clarification: this covers normal discovery; POAP/preprovision uses NDFC_SW_DISCOVERY_*

Validation

• SHA_AES validated end-to-end through the full NaC pipeline (validate → create → deploy → idempotency) against NDFC 4.2.1.10 with NX-OS 10.6.
• No schema, code, or default values changed.

Test plan

• Review example renders correctly on GitHub
• Verify cross-reference links in both files resolve correctly
• Confirm NX-OS commands are syntactically correct for NX-OS 10.x

Closes #871