Embed DartPad in the top part of homepage

src/tests/site/gists/README.md

+This directory contains source code for some examples published
+using [DartPad](https://github.com/dart-lang/dart-pad).
+To generate gists suitable for DartPad, we use
+[kasperpeulen's gist-generator](https://github.com/kasperpeulen/gist-generator).
+See Kasper's project for the latest instructions on using the tool,
+but here's what we do, for now.
+
+To get the tool:
+
+```
+pub global activate --source git https://github.com/kasperpeulen/gist-generator
+```
+
+To test whether all the examples follow DartPad's format,
+without actually generating any gists:
+
+```
+<from a directory containing one or more packages>
+gist generate -n
+```
+
+To generate a trial version of the gists, without changing any
+gists that are already published:
+
+```
+gist generate -t
+```
+
+To generate or update all gists for packages under the current directory:
+
+```
+gist generate
+```
+
+For that last command to work, you'll need the gist token for whichever
+user all the gists are under. (Contact the user.) If that is impractical,
+an alternative is to edit each pubspec.yaml, removing the `gist:` and
+`dartpad:` entries at the end. Then run `gist generate` and update all
+references to point to the new locations.