Add note on ModelInfo for Gemini Models

[victordibia]

Why are these changes needed?

Add note on how to update modelinfo for new models.

Related issue number

Closes #6258

Checks

• I've included any doc changes needed for https://microsoft.github.io/autogen/. See https://github.com/microsoft/autogen/blob/main/CONTRIBUTING.md to build and test documentation locally.
• I've added tests (if relevant) corresponding to the changes introduced in this PR.
• I've made sure all auto checks have passed.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 77.27%. Comparing base (7abbdc8) to head (5d3b168).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6259      +/-   ##
==========================================
+ Coverage   73.01%   77.27%   +4.26%     
==========================================
  Files         306      200     -106     
  Lines       17440    14322    -3118     
  Branches      406        0     -406     
==========================================
- Hits        12733    11068    -1665     
+ Misses       4434     3254    -1180     
+ Partials      273        0     -273     

Flag	Coverage Δ
unittests	77.27% <ø> (+4.26%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.