docs: add README for Dart multi_counter sample

[jhuleatt]

Adds a README.md to the Dart/multi_counter sample with information about the Cloud Next 2026 talk and the session link. It also adds a reference to the sample in the root README.md.

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PR created automatically by Jules for task 2165079252949837490 started by @jhuleatt

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[gemini-code-assist]

Code Review

This pull request introduces documentation for a new 'Multi-counter' sample, featuring a Flutter application and Dart-based Cloud Functions for Firebase. Feedback was provided to ensure the project descriptions in both the main and sample-specific README files accurately reflect the inclusion of a shared Dart package used by both the client and server components.