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/layout.tsx

@@ -0,0 +1,7 @@
+import { pageMetadata } from "@/lib/page-metadata";
+
+export const metadata = pageMetadata("providers/browser-use");
+
+export default function Layout({ children }: { children: React.ReactNode }) {
+  return children;
+}