Add top-level crate documentation/READMEs (#12907)

# Description
Add `README.md` files to each crate in our workspace (-plugins) and also
include it in the `lib.rs` documentation for <docs.rs> (if there is no
existing `lib.rs` crate documentation)

In all new README I added the defensive comment that the crates are not
considered stable for public consumption. If necessary we can adjust
this if we deem a crate useful for plugin authors.

crates/nu-engine/README.md

@@ -0,0 +1,9 @@
+This crate primarily drives the evaluation of expressions.
+
+(Some overlap with nu-protocol)
+
+- Provides `CallExt`
+
+## Internal Nushell crate
+
+This crate implements components of Nushell and is not designed to support plugin authors or other users directly.

crates/nu-engine/src/lib.rs

@@ -1,3 +1,4 @@
+#![doc = include_str!("../README.md")]
 mod call_ext;
 mod closure_eval;
 pub mod column;