Remove mostly duplicated web-specific get started codelab (#11912)

This version of the getting started codelab hasn't been updated in a lot
time and with Filip's ["Your first Flutter app"
codelab](https://docs.flutter.dev/get-started/codelab), it's mostly
duplicated content. We prefer developers following that one and
according to analytics, they mostly do.

This PR then redirects from the removed page to the preferred codelab's
introduction.

Contributes to #11911 by
starting to clean up the getting started section and reducing
duplication of the content that it will add.

Author: parlough

.github/dependabot.yml

@@ -133,7 +133,6 @@ updates:
       - "/examples/data-and-backend/json"
       - "/examples/deployment/obfuscate"
       - "/examples/example_utils"
-      - "/examples/get-started/codelab_web"
       - "/examples/get-started/flutter-for/android_devs"
       - "/examples/get-started/flutter-for/declarative"
       - "/examples/get-started/flutter-for/ios_devs"