Recreate `dart-lang.github.io/markdown`

[mosuem]

After merging package:markdown into this monorepo, we should find a solution to host the example at https://dart-lang.github.io/markdown.

cc @devoncarew

[devoncarew]

Re-opening as we should set up a redirect from the old site via https://github.com/dart-archive/dart-lang.github.io.

[devoncarew]

Live at https://dart-lang.github.io/tools/markdown/ !

[devoncarew]

dart-lang/dart-lang.github.io#5 (for the redirect)

[devoncarew]

I think this is set up to work once the dart-lang/markdown repo is archived (and moves to dart-archive).