docs: add a page for the PPDS Skills (AI agent) package

[joshsmithxrm]

Context

The ppds-skills package is a knowledge layer that teaches AI coding agents to drive PPDS well — its reference tables are generated verbatim from ppds --help to stop flag hallucination. As PPDS leans into agent/MCP usage, the docs site should describe it and how to install it.

Ask

Add a "Skills / AI agents" page (or a section under the MCP/agent docs) covering:

• what the skills are and the captured-help anti-hallucination approach
• install for Claude Code (plugin marketplace) and GitHub Copilot CLI (skill dirs)
• a link to the repo

This also supports the passive-discoverability work (topics, llms.txt) tracked in joshsmithxrm/power-platform-developer-suite#1211.

Notes

• Action once ppds-skills is public.

Acceptance criteria

• A docs page documents the skills package and links the repo.