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/test/yaml/bad.yaml

@@ -0,0 +1,27 @@
+samples:
+  # Bad type, should be string, but it's a number.
+  - name: 42
+    screenshots:
+      - url: https://placekitten.com/500/500
+        alt: a kitten
+      - url: https://placekitten.com/400/400
+        alt: another kitten
+    source: http://github.com/johnpryan/kittens
+    description: A sample kitten app
+    difficulty: beginner
+    widgets:
+      - AnimatedBuilder
+      - FutureBuilder
+    packages:
+      - json_serializable
+      - path
+    tags: ['beginner', 'kittens', 'cats']
+    platforms: ['web', 'ios', 'android']
+    links:
+      - text: inspiration
+        href: https://apps.apple.com/us/app/neko-atsume-kitty-collector/id923917775
+      - text: author
+        href: http://jpryan.me
+    type: sample # sample, app, or cookbook
+    date: 2019-12-15T02:59:43.1Z
+    channel: stable

web/samples_index/test/yaml/single.yaml

@@ -0,0 +1,26 @@
+samples:
+  - name: Kittens
+    screenshots:
+      - url: https://placekitten.com/500/500
+        alt: a kitten
+      - url: https://placekitten.com/400/400
+        alt: another kitten
+    source: http://github.com/johnpryan/kittens
+    description: A sample kitten app
+    difficulty: beginner
+    widgets:
+      - AnimatedBuilder
+      - FutureBuilder
+    packages:
+      - json_serializable
+      - path
+    tags: ['beginner', 'kittens', 'cats']
+    platforms: ['web', 'ios', 'android']
+    links:
+      - text: inspiration
+        href: https://apps.apple.com/us/app/neko-atsume-kitty-collector/id923917775
+      - text: author
+        href: http://jpryan.me
+    type: sample # sample, app, or cookbook
+    date: 2019-12-15T02:59:43.1Z
+    channel: stable