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-cli/README.md

@@ -0,0 +1,7 @@
+This crate implements the core functionality of the interactive Nushell REPL and interfaces with `reedline`.
+Currently implements the syntax highlighting and completions logic.
+Furthermore includes a few commands that are specific to `reedline`
+
+## Internal Nushell crate
+
+This crate implements components of Nushell and is not designed to support plugin authors or other users directly.

crates/nu-cli/src/lib.rs

@@ -1,3 +1,4 @@
+#![doc = include_str!("../README.md")]
 mod commands;
 mod completions;
 mod config_files;