Add DartPad to flutter.dev (#2938)

* Added dartpad but with old examples from dart.dev

* Updated dartpad to flutter mode + light theme

* Refactor to counter example

* Added 3 working examples, added dartpad to install page

* Add an include in index

* Add style and language updates to /docs/get-started/install:

- Reframe the installation page as the very first step of get started (aka try/install)
- Put the embedded Dartpad above the installation buttons and change the page title.
- Update title in left nav

* Minor updates/corrections

* Fix broken link for chrome os

* Add styling tweak to dartpad section on landing page

* Use 'Install Flutter' on landing page

* Fix typo

* Rebuild dartpad_picker

* Remove warning about Chrome 77

* Left-align headings on install page

* Remove changes to install page

* Remove changes to sidenav for install page

.gitignore

@@ -10,7 +10,6 @@ node_modules
 .flutter-plugins
 .packages
 .dart_tool
-# pubspec.lock
 example.g
 tmp/
 

_config.yml

@@ -116,6 +116,17 @@ alert:
  <i class="fas fa-exclamation-triangle"></i> **Warning:**
  end: </aside>
 
+custom:
+ dartpad:
+ embed-dart-prefix: https://dartpad.dev/embed-dart.html
+ embed-html-prefix: https://dartpad.dev/embed-html.html
+ embed-inline-prefix: https://dartpad.dev/embed-inline.html
+ dartpadx:
+ embed-dart-prefix: https://dartpad.dev/experimental/embed-new-dart.html
+ embed-flutter-prefix: https://dartpad.dev/experimental/embed-new-flutter.html
+ embed-html-prefix: https://dartpad.dev/experimental/embed-new-html.html
+ embed-inline-prefix: https://dartpad.dev/experimental/embed-new-inline.html
+
  # callout_danger: '<div class="bs-callout bs-callout-danger">'
  # callout_default: '<div class="bs-callout bs-callout-default">'
  # callout_primary: '<div class="bs-callout bs-callout-primary">'