docs: move platform matrices to README.md

[russellwheatley]

Description

The links are broken for the "firebase-logo.png" image, but it should work once merged with master branch as it is looking for the image on master branch. I tested with an existing image in .github/images and it worked:

Related Issues

none.

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. Updating the pubspec.yaml and changelogs is not required.

• [ ] 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 (melos run analyze) does not report any problems on my PR.
• [ ] I read and followed the Flutter Style Guide.
• [ ] 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.
• [X] No, this is not a breaking change.

[kroikie]

This is sweet! Would it be possible to split mobile into iOS and Android for a bit more clarity?