* docs: add setup guides for ChatGPT Pro, GitHub Copilot, and Qwen Code
Add individual OAuth setup guide pages with step-by-step screenshots
for each provider. Add "Use Your Existing Subscription" section to the
Bring Your Own LLM page with card links to each guide. Register pages
in docs navigation.
* docs: add ChatGPT Pro setup screenshots
* docs: use custom provider icons for OAuth setup cards
* docs: inline SVG icons in provider cards for dark mode support
* docs: place provider icons above card titles
