docs: add in-code docs for FileGroup

[xushiyan]

Description

Updated based on #214

How are the changes test-covered

• N/A
• Automated tests (unit and/or integration tests)
• Manual tests
  • Details are described below

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 91.02%. Comparing base (884a7f8) to head (e6a1f0c).
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #269   +/-   ##
=======================================
  Coverage   91.02%   91.02%           
=======================================
  Files          41       41           
  Lines        2050     2050           
=======================================
  Hits         1866     1866           
  Misses        184      184           

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