docs: Add comprehensive crate and module documentation
- Add detailed crate-level documentation with project overview, architecture, and feature descriptions
- Document all major modules: HTTP handlers, storage, AT Protocol, i18n, config, filtering, OAuth, validation, and JOSE
- Fix HTML tag warnings in error documentation by escaping angle brackets
- Resolve broken intra-doc links by replacing wildcard references with actual function names
- Generate clean documentation with zero warnings using `cargo doc`
Documentation now includes technology stack details, module organization charts,
usage examples, and cross-references to improve developer experience and
contribution onboarding.