docs: add comprehensive context files documentation

[kucukkanat]

• Add section explaining automatic context file loading
• Document supported file types and naming conventions
• Include configuration examples for custom context paths
• Add technical details section with implementation specifics
• Reference key source files for context processing system

This documentation helps users understand how Crush automatically loads project-specific instructions and preferences to guide AI agent behavior.

Relates (but does not resolve) to the Issue: Load user-level memory/context files #1050

• I have read CONTRIBUTING.md.
• I have created a discussion that was approved by a maintainer (for new features).

[charmcli]

All contributors have signed the CLA ✍️ ✅
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[kucukkanat]

I have read the Contributor License Agreement (CLA) and hereby sign the CLA.

[kucukkanat]

Hi @caarlos0, @andreynering, and @kujtimiihoxha!
Could you please take a look at this PR and share your feedback when you get a chance? Your input is always appreciated, and it would be great to have your review on the comprehensive context files documentation. Thanks so much for your time!