Add Docs folder and basic structure for documentation.

Author: zzap

docs/overview.md

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+# WP-CLI AI Command Documentation
+
+## Introduction
+
+### Dependencies
+
+## Problem
+
+## Key features
+
+### Resources
+
+#### REST controllers as MCP tools
+
+### Client
+
+### Server
+

docs/reference.md

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+# Reference
+
+## Command
+
+## Client
+
+## Server

docs/tools.md

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+# Available tools
+
+## Predefined Tools
+
+## REST Controllers