Add samples index (#359)

* add samples_index to web/ directory

Co-authored-by: Thea Flowers <theaflowers@google.com>

* add pub_get.dart script

* build sample index in peanut post build

* re-generate sample index with web demos

* print more details in peanut_post_build.dart

* add images for demos

* run generator

* update README

* add animations and provider shopper as symlinks

* add links to symlinked web demos

* use relative paths

* update cookbook images, urls, and description CSS

* use relative URL for navbar link

* unstage HTML files

* .gitignore generated HTML files

* add margin to toolbar

* rename escape functions

* add and update copyright headers

Co-authored-by: Thea Flowers <theaflowers@google.com>

web/samples_index/pubspec.yaml

@@ -0,0 +1,27 @@
+name: samples_index
+description: A visual index of Flutter samples
+homepage: https://github.com/flutter/samples_index
+author: Flutter Team <flutter-dev@googlegroups.com>
+version: 0.0.1
+environment:
+  sdk: '>=2.5.0 <3.0.0'
+dependencies:
+  json_annotation: ^3.0.0
+  path: ^1.6.0
+  resource: ^2.1.6
+  yaml: ^2.2.0
+  mdc_web: ^0.5.0-pre
+  sass_builder: ^2.1.0
+  checked_yaml: ^1.0.0
+  webdriver: ^2.1.0
+  html: ^0.14.0
+dev_dependencies:
+  grinder: ^0.8.3
+  pedantic: ^1.8.0
+  test: ^1.6.0
+  json_serializable: ^3.2.0
+  build: ^1.2.0
+  build_runner: ^1.7.0
+  build_web_compilers: ^2.7.0
+  tuneup: ^0.3.6
+