Redirect samples index url to docs site

[ericwindmill]

Set up a redirect from flutter.github.io/samples to the new page on the docs site.

Pre-launch Checklist

• I read the [Contributor Guide] and followed the process outlined there for submitting PRs.
• I read the [Tree Hygiene] wiki page, which explains my responsibilities.
• I read the [Flutter Style Guide] recently, and have followed its advice.
• I signed the [CLA].
• I listed at least one issue that this PR fixes in the description above.
• I updated/added relevant documentation (doc comments with ///).
• I added new tests to check the change I am making, or this PR is [test-exempt].
• All existing and new tests are passing.

[Piinks]

LGTM