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-color-config/README.md

@@ -0,0 +1,5 @@
+Logic to resolve colors for syntax highlighting and output formatting
+
+## Internal Nushell crate
+
+This crate implements components of Nushell and is not designed to support plugin authors or other users directly.

crates/nu-color-config/src/lib.rs

@@ -1,3 +1,4 @@
+#![doc = include_str!("../README.md")]
 mod color_config;
 mod matching_brackets_style;
 mod nu_style;