Improve development guide

[simonsan]

Currently, this is being empty: https://github.com/rustic-rs/rustic/blob/main/docs/dev/development_guide.md

As a starter, we should explain the following things:

• how to use just for common commands (#820)
• cargo xtask for coverage reports and installing needed dependencies (#820)
• setting up an IDE (can help with VSCodium) for Rust development
• link to ARCHITECTURE.md (#757) when ready, so devs can get a fast grip to what is where
• tbc