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/README.md

@@ -0,0 +1,43 @@
+# Flutter samples index generator
+
+This tool is used to generate the visual samples index for Flutter samples.
+
+## Generating the index
+
+We use [grinder](https://pub.dev/packages/grinder) to run the build tasks:
+
+```bash
+$ pub get
+$ pub global activate grinder
+$ grind build
+```
+
+This will generate the index into `./web`
+
+## Serving the index locally
+
+If you want to serve the index locally, you can use
+[webdev](https://pub.dev/packages/webdev):
+
+```bash
+$ webdev serve
+```
+
+## Publishing the index
+
+You can build the complete index into a publishable directory using Grinder:
+
+```bash
+$ grind build-release
+```
+
+This outputs the completely built index to `./public`.
+
+## Generating cookbook content
+
+The cookbook articles are generated using a WebDriver script that scrapes the
+flutter.dev website. To run:
+
+1. Install [ChromeDriver](https://chromedriver.chromium.org/downloads)
+2. run `chromedriver --port=4444 --url-base=wd/hub --verbose`
+3. run `grind scrape-cookbook`