Document passing arguments to external commands

[ColumbusLabs]

What

Document how Nushell passes arguments to external commands.

Why

The existing page explains how to select an external command, but not how quoted strings, lists, or dynamic executable names map to the invocation.

Fixes #527

Details

• explains that quoted spaces stay inside a single argument
• makes the direct-list error explicit and shows ...$paths for separate arguments
• documents executable names stored in variables with ^$program
• links the canonical spread, string-command, and extern documentation

Testing

• npm ci
• npm run build (1,628 pages rendered)
• npx prettier --check book/running_externals.md
• rendered-link and anchor inspection
• git diff --check

[fdncred]

Thanks