Add clear documentation explaining that when using the default GitHub app:
- Device flow is always enabled
- Setting CODER_OAUTH2_GITHUB_DEVICE_FLOW=false will be ignored
- This behavior only applies to the default GitHub app

Fixes #16824

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

## Issue

Closes #16824

Document that the default GitHub authentication app provided by Coder
requires device flow, and that this behavior cannot be overridden.

## Changes Made

Claude updated the GitHub authentication documentation to:

1. Add a prominent warning in the Default Configuration section
explaining that the default GitHub app requires device flow and ignores
the `CODER_OAUTH2_GITHUB_DEVICE_FLOW` setting
2. Clarify the Device Flow section to indicate that:
   - Device flow is always enabled for the default GitHub app
   - Device flow is optional for custom GitHub OAuth apps
- The `CODER_OAUTH2_GITHUB_DEVICE_FLOW` setting is ignored when using
the default app


[preview](https://coder.com/docs/@16824-github-device-flow/admin/users/github-auth)

<sub>🤖 Generated with [Claude Code](https://claude.ai/code)</sub>

---------

Co-authored-by: EdwardAngert <17991901+EdwardAngert@users.noreply.github.com>
Co-authored-by: Claude <noreply@anthropic.com>
Co-authored-by: M Atif Ali <atif@coder.com>