docs: create providers/ section with dedicated provider pages (#928)

Add dedicated documentation pages for each cloud browser provider:
Browser Use, Browserbase, Browserless, and Kernel (sorted
alphabetically). Uses providers/ path per maintainer feedback.

- Fix Kernel defaults to match source code (KERNEL_HEADLESS=true,
  KERNEL_STEALTH=false) — README had these inverted
- Add Providers section to sidebar navigation
- Simplify cdp-mode cloud providers section to link to new pages

Fixes part of #774

Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

docs/src/app/providers/browser-use/page.mdx

@@ -0,0 +1,24 @@
+# Browser Use
+
+[Browser Use](https://browser-use.com) provides cloud browser infrastructure for AI agents. Use it when running agent-browser in environments where a local browser isn't available (serverless, CI/CD, etc.).
+
+## Setup
+
+```bash
+export BROWSER_USE_API_KEY="your-api-key"
+agent-browser -p browseruse open https://example.com
+```
+
+Or use environment variables for CI/scripts:
+
+```bash
+export AGENT_BROWSER_PROVIDER=browseruse
+export BROWSER_USE_API_KEY="your-api-key"
+agent-browser open https://example.com
+```
+
+The `-p` flag takes precedence over `AGENT_BROWSER_PROVIDER`.
+
+When enabled, agent-browser connects to a Browser Use cloud session instead of launching a local browser. All commands work identically.
+
+Get your API key from the [Browser Use Cloud Dashboard](https://cloud.browser-use.com/settings?tab=api-keys). Free credits are available to get started, with pay-as-you-go pricing after.