Fix: feat: firebase authentication guide #2

[ChunhThanhDe]

Description

Fix #2
Create Example for firebase authentication with flutter
Create Doc Guide for this in README

Type of Change

• 🚀 New feature (non-breaking change which adds functionality)
• 🐞 Bug fix (non-breaking change which fixes an issue)
• ⚠️ Breaking change (fix or feature that would cause existing functionality to change)
• 🔄 Code refactor
• ⚙️ Build configuration change
• 📝 Documentation
• 🗑️ Chore

How Has This Been Tested?

Please describe the tests that you ran to verify your changes. Provide instructions so we can reproduce.
Please also list any relevant details for your test configuration

• Test Create new Account
• Login, signout Account

Pre-launch Checklist:

• I have read the Contributor Guide and followed the process for submitting PRs.
• I have followed the Flutter Style Guide, including widget expectations.
• I have signed the CLA.
• I have listed at least one issue that this PR fixes.
• I updated or added relevant documentation.
• Is the code as simple and readable as possible?
• I added tests for the changes I made (or marked as test-exempt).
• I have ensured that all tests are passing.