Point opensource site at new location

[samtstern]

Description

This page currently gets about 1000 views/week:
https://firebaseopensource.com/projects/flutter/plugins/

While I am removing it from the firebaseopensource.com home page, I expect there will be some lingering direct traffic to the page. Rather than having it 404, I'd like it to send people to the new location.

Related Issues

firebase/flutterfire#127

Checklist

Before you create this PR confirm that it meets all requirements listed below by checking the relevant checkboxes ([x]). This will ensure a smooth and quick review process.

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• My PR includes unit or integration tests for all changed/updated/fixed behaviors (See Contributor Guide).
• All existing and new tests are passing.
• I updated/added relevant documentation (doc comments with ///).
• The analyzer (flutter analyze) does not report any problems on my PR.
• I read and followed the Flutter Style Guide.
• The title of the PR starts with the name of the plugin surrounded by square brackets, e.g. [shared_preferences]
• I updated pubspec.yaml with an appropriate new version according to the pub versioning philosophy.
• I updated CHANGELOG.md to add a description of the change.
• I signed the CLA.
• I am willing to follow-up on review comments in a timely manner.

Breaking Change

Does your PR require plugin users to manually update their apps to accommodate your change?

• Yes, this is a breaking change (please indicate a breaking change in CHANGELOG.md and increment major revision).
• No, this is not a breaking change.

[samtstern]

cc @kroikie @collinjackson

[samtstern]

New page is live:
https://firebaseopensource.com/projects/flutter/plugins/