Document `dart run <package>@` syntax for dynamic remote execution

[kevmoo]

The dart run <package>@[<descriptor>] syntax was recently introduced (unifying descriptor parsing, see dart-lang/sdk#62123) to allow dynamic, one-off execution of remote packages from pub.dev, similar to npx in the Node ecosystem.

Currently, this feature is completely undocumented on dart.dev.

Specifically:

1. src/content/tools/dart-run.md: This page currently explicitly directs users to the legacy command: "To run a program from an arbitrary location, use the pub global command." This should be updated to explain the dart run pkg@ syntax.
2. Tutorials / Guides: High-traffic guides like dart-doc.md that currently instruct users to install one-off servers (e.g., dart install dhttpd) should be updated to recommend dart run dhttpd@ to avoid polluting the user's PATH with temporary tools.

[harshyadavDeveloper]

I've opened a PR that addresses this by documenting the dart run <package>@[<descriptor>] syntax in dart-run.md and updating the dart-doc.md guide to use the new one-off execution syntax for dhttpd.

Looking forward to any feedback.