[cloud_firestore, cloud_firestore_web] Update Firestore web documentation.

[ditman]

Description

• Mark cloud_firestore and cloud_functions as being Web ready in main README support matrix.
• Add more detailed setup instructions about the Web platform on READMEs.
• Update install instructions post-federation.

Related Issues

flutter/flutter#45293

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.
• If the pull request affects only one plugin, the PR title starts with the name of the plugin in brackets (e.g. [cloud_firestore])
• 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.
• 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?

• No, this is not a breaking change.

[ditman]

Merged and published:

• https://pub.dev/packages/cloud_firestore/versions/0.13.2+1
• https://pub.dev/packages/cloud_firestore_web/versions/0.1.0+2