Add llms.txt for Partner API v2 docs

[samuelwisemanhx]

What does this PR do? (please provide any background)

Adds an llms.txt file at /partner/v2/llms.txt following the llmstxt.org specification. This gives LLMs a curated entry point into the v2 (APIConnect) documentation, so partners using AI assistants to build their integration get better answers.

The file links to:

• Getting started docs (overview, auth, endpoint navigation guide, OpenAPI schema)
• Webhook integration guide
• All v2 problem/error type pages

Any tech debt?

No. Could add an llms-full.txt later that inlines all page content for single-fetch LLM consumption.

Screenshots / Screencast

N/A — static text file, no UI changes.

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