Introduce a tutorial about fetching data using package:http (#4520)

This replaces the old Dart Fetch data dynamically article which is web-based and not platform independent.

This PR introduces a completely new tutorial built around `package:http` allowing the logic to work to work on multiple platforms, including for CLI, web, and Flutter apps.

The tutorial uses mock package information data for its examples, hosted directly on the site.

Closes #3852

Co-authored-by: Shams Zakhour (ignore Sfshaza) <44418985+sfshaza2@users.noreply.github.com>

Author: parlough

_config.yml

@@ -123,3 +123,9 @@ alert:
     <aside class="alert alert-warning" role="alert" markdown="1">
     <i class="fas fa-exclamation-triangle"></i> **Warning:**
   end: </aside>
+
+why:
+  learn: >-
+    <div class="mini-toc" markdown="1">
+      <h4 class="no_toc">What you'll learn</h4>
+  end: </div>