Add Documentation Generation Feature to Speckit for Standardized Project Documentation

## Description
As a developer working on complex projects, I've recently created standardized documentation templates to ensure consistency across system modules. This includes user guides, API references, and Mermaid diagram collections. I propose adding a documentation generation feature to Speckit that can automatically scaffold and populate these documentation artifacts based on project structure and code analysis.

## Proposed Feature
Implement a speckit docs generate command or similar functionality that:
Analyzes the project structure (e.g., C# assemblies, Unity scripts)
Generates standardized Markdown templates for:
User guides (task-oriented, with quick starts and common scenarios)
API references (minimal, table-format parameter docs with examples)
Diagram collections (Mermaid-based class diagrams, flowcharts, sequence diagrams, state diagrams)
Populates templates with extracted code metadata (e.g., namespaces, method signatures, class relationships)
Supports configuration files for customization (e.g., diagram complexity levels based on system scale)

## Differentiation from Existing Speckit Documentation Features
While Speckit may already provide basic documentation generation (such as data-model.md or quick-start.md), this proposed feature differs by focusing on comprehensive, user-facing documentation suites rather than developer-only artifacts. It emphasizes structured templates for end-user guides and visual diagrams, enabling better onboarding and system understanding, whereas existing features might prioritize code-level documentation. This addition would complement Speckit's current capabilities by bridging the gap between technical API docs and practical user documentation.