Add content on `sqlx`

[hdoordt]

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