chore(docs): add metadata and use core error

[mroczect]

Summary

This PR adds standard Cargo package metadata to the docs crate and switches the main function's error type from std::error::Error to core::error::Error.

Motivation & Context

Adding metadata improves crate discoverability and documentation. Using core::error::Error aligns the docs application with the library's no_std-friendly error handling patterns and avoids unnecessary reliance on the standard library.

Detailed Changes

• docs/Cargo.toml: Added description, license, repository, readme, keywords, and categories. Reformatted the librawssg dependency line for consistency.
• docs/src/main.rs: Changed fn main() -> Result<(), Box<dyn std::error::Error>> to fn main() -> Result<(), Box<dyn core::error::Error>>.

Impact & Breaking Changes

No breaking changes identified.

Testing Recommendations

Run cargo run -p docs to ensure the documentation generator still builds and runs correctly.

Related Issues

None.