Clarify that it's the ZF discord and that it's engineering-focused.
//!
//! [Rendered docs from the `main` branch](https://doc.zebra.zfnd.org).
//!
-//! [Join us on Discord](https://discord.gg/na6QZNd).
+//! [Join us on the Zcash Foundation Engineering Discord](https://discord.gg/na6QZNd).
#![doc(html_logo_url = "https://www.zfnd.org/images/zebra-icon.png")]
#![doc(html_root_url = "https://doc.zebra.zfnd.org/zebrad")]
