Remove layout basics codelab (flutter#9876)

We currently have three locations introducing or exploring layout basics
in Flutter: [Layouts in Flutter](https://docs.flutter.dev/ui/layout),
[Building layouts
tutorial](https://docs.flutter.dev/ui/layout/tutorial), and this [Basic
Flutter layout concepts
codelab](https://docs.flutter.dev/codelabs/layout-basics).

We need a better and consolidated layout story. I think rather than
hobbling along with these three pieces of outdated and overlapping
material, we need to start from a cleaner place where users don't have
so many unnecessary choices. As a first step, I think we need to just
remove one of these, and I think this codelab is a good choice to start
with for a few reasons:

- The DartPad technology (testing, solutions, etc) it relies on is going
away soon
- @johnpryan This page is the main user of DartPad testing functionality
on the Flutter site.
- The many embedded DartPads result in a slow site loading experience
and an essential non-functional mobile experience
- The code on the page does not have excerpts and has been outdated,
broken, or filled with warnings for surprising amounts of time multiple
times in the past
- The formatting of the page and text is inconsistent with the rest of
the site
- With little teaching around each layout type, the API docs are likely
a better resource and often have runnable snippets

This type of learning experience should eventually be replaced by a
consolidated interactive step-by-step in-browser learning experience for
intro Flutter concepts.

Contributes to flutter#9738
Closes flutter#6300

firebase.json

@@ -101,6 +101,7 @@
       { "source": "/accessibility-and-localization", "destination": "/ui/accessibility-and-internationalization", "type": 301 },
       { "source": "/accessibility-and-localization/:rest*", "destination": "/ui/accessibility-and-internationalization/:rest*", "type": 301 },
       { "source": "/add-to-app/android/add-splash-screen", "destination": "/platform-integration/android/splash-screen", "type": 301 },
+      { "source": "/codelabs/layout-basics", "destination": "/ui/layout", "type": 301 },
       { "source": "/codelabs/explicit-animations", "destination": "/ui/animations/tutorial", "type": 301 },
       { "source": "/cookbook/games", "destination": "https://flutter.dev/games", "type": 301 },
       { "source": "/cookbook/games/google-mobile-ads", "destination": "/cookbook/plugins/google-mobile-ads", "type": 301 },

src/codelabs/index.md

@@ -84,10 +84,6 @@ one of the following codelabs:
 Learn about Material Design and basic Flutter concepts,
 like layout and animations:
 
-* [Basic Flutter layout concepts][]<br>
-  Use DartPad in a browser (no downloads required!)
-  to learn the basics of creating a Flutter layout.
-
 * [How to debug layout issues with the Flutter Inspector][]<br>
   Not an official codelab, but step-by-step instructions on
   how to debug common layout problems using the Flutter 
@@ -145,7 +141,6 @@ like layout and animations:
 [Building next generation UIs in Flutter]: {{site.codelabs}}/codelabs/flutter-next-gen-uis#0
 [Adaptive Apps in Flutter]: {{site.codelabs}}/codelabs/flutter-adaptive-app
 [animations]: {{site.pub}}/packages/animations
-[Basic Flutter layout concepts]: {{site.url}}/codelabs/layout-basics
 [Building Beautiful Transitions with Material Motion for Flutter]: {{site.codelabs}}/codelabs/material-motion-flutter
 [Building scrolling experiences in Flutter]: {{site.youtube-site}}/watch?v=YY-_yrZdjGc
 [How to debug layout issues with the Flutter Inspector]: {{site.flutter-medium}}/how-to-debug-layout-issues-with-the-flutter-inspector-87460a7b9db