Replace #8824: CONTRIBUTING.md for standard library (#8894)

# Description
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Replaces #8824, which was languishing in review limbo and becoming
increasingly difficult to keep current with upstream changes.

In addition to all the edits, this PR includes updated documentation for
running unit tests via `std run-tests`.

# User-Facing Changes
<!-- List of all changes that impact the user experience here. This
helps us keep track of breaking changes. -->
A CONTRIBUTING.md documenting guidelines and getting started info for
potential stdlib contributors.
# Tests + Formatting
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> **Note**
> from `nushell` you can also use the `toolkit` as follows
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> use toolkit.nu # or use an `env_change` hook to activate it
automatically
> toolkit check pr
> ```
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# After Submitting
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crates/nu-std/src/lib.rs

@@ -79,6 +79,13 @@ pub fn load_standard_library(
             ("xml", include_str!("../lib/xml.nu")),
         ];
 
+        // Define commands to be preloaded into the default (top level, unprefixed) namespace.
+        // User can invoke these without having to `use std` beforehand.
+        // Entries are: (name to add to default namespace, path under std to find implementation)
+        //
+        // Conventionally, for a command implemented as `std foo`, the name added
+        // is either `std foo` or bare `foo`, not some arbitrary rename.
+
         let prelude = vec![
             ("std help", "help"),
             ("std help commands", "help commands"),