Add feature documentation generation skill

[mariamKh]

This pull request adds a new skill definition for generating feature documentation. The skill, described in .claude/skills/generate-feature-docs/SKILL.md, outlines a structured process for classifying features, handling screenshots, generating documentation, and auto-linking related docs.

Key additions and workflow details:

Skill definition and workflow:

• Introduces the generate-feature-docs skill, which automates documentation generation under user-docs/<featureName>, including classification (backend, frontend, full-stack), screenshot handling, and related doc linking.

Feature classification and screenshot handling:

• Details a step-by-step process to classify the feature based on code evidence, and to capture or use existing screenshots as appropriate for frontend or full-stack features, while providing clear fallback behavior for backend-only features.

Documentation generation:

• Specifies the creation of two Markdown files—implementation-details.md and user-guide.md—with tailored content based on the feature type, including API references, workflow explanations, and usage guides.

Auto-linking related documentation:

• Outlines a method for detecting and linking genuinely related documentation from other features or repo-level docs, with strict criteria to avoid spurious connections.