Add How To Code It walkthroughs

[AngusGMorrison]

Adds three recent pieces from How To Code It:

• Rust In Space!, my RustConf talk, in which Rustaceans learn to model 13 billion years of time – with femtosecond precision – in seven astronomical timescales.
• Part IV of Master Hexagonal Architecture in Rust, a deep dive into the trade-offs of adopting hexagonal architecture.
• Part III of The Definitive Guide to Error Handling, where we take an exhaustive look at structured error handling with examples from around the Rust ecosystem.

[mariannegoldin]

Great articles, TY very much!

[AngusGMorrison]

Thanks, @mariannegoldin 🙏