docs: add tested Browser Use setup inside containers

[MagMueller]

Add a Browser Use guide and one navigation entry. The coding agent uses Chrome inside its Container Use environment instead of a host desktop browser.

The setup explicitly installs uv: browser-use install calls uvx, which a pip-only setup does not provide. It then starts headless Chrome inside the container and points the CLI at its local CDP endpoint with BU_CDP_URL.

Validation against main 2e43e62:

• Fresh python:3.12-bookworm with pip install browser-use: browser-use install fails with FileNotFoundError for uvx.
• Corrected container cohort with uv and browser-use 0.13.10 (browser-harness CLI 0.1.13): Chromium installs; the exact guide starts Chrome and returns title Example Domain with URL https://example.com/.
• Mintlify build validation and git diff --check pass.

Only docs/browser-use.mdx and one docs/docs.json navigation line change. No runtime, default image, or secret handling changes. The no-sandbox flag is limited to Chrome inside the isolated container, not the host. Both test containers were removed after execution.