Add content on `sqlx`

Add content on the `sqlx` crate with Postgres

- How to set up
- Migrations
- Best practices

Best would be to contribute it directly to the `restructure` branch while it's still unmerged. There's a file on that branch where the content could be added: https://github.com/tweedegolf/101-rs/blob/restructure/slides/D-rust-for-web/sqlx.md