docs: refactor README by moving sections to docs/ folder

Move 11 documentation sections to separate markdown files in docs/:
- models.md, load-balancing.md, web-console.md, configuration.md
- menubar-app.md, api-endpoints.md, testing.md, troubleshooting.md
- safety-notices.md, legal.md, development.md

README now contains a Documentation section with links to each doc.
Also moved donation link to above Star History section.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

docs/configuration.md

@@ -0,0 +1,18 @@
+# Advanced Configuration
+
+While most users can use the default settings, you can tune the proxy behavior via the **Settings → Server** tab in the WebUI or by creating a `config.json` file.
+
+## Configurable Options
+
+- **API Key Authentication**: Protect `/v1/*` API endpoints with `API_KEY` env var or `apiKey` in config.
+- **WebUI Password**: Secure your dashboard with `WEBUI_PASSWORD` env var or in config.
+- **Custom Port**: Change the default `8080` port.
+- **Retry Logic**: Configure `maxRetries`, `retryBaseMs`, and `retryMaxMs`.
+- **Rate Limit Handling**: Comprehensive rate limit detection from headers and error messages with intelligent retry-after parsing.
+- **Load Balancing**: Adjust `defaultCooldownMs` and `maxWaitBeforeErrorMs`.
+- **Persistence**: Enable `persistTokenCache` to save OAuth sessions across restarts.
+- **Max Accounts**: Set `maxAccounts` (1-100) to limit the number of Google accounts. Default: 10.
+- **Quota Threshold**: Set `globalQuotaThreshold` (0-0.99) to switch accounts before quota drops below a minimum level. Supports per-account and per-model overrides.
+- **Endpoint Fallback**: Automatic 403/404 endpoint fallback for API compatibility.
+
+Refer to `config.example.json` for a complete list of fields and documentation.