docs: add WSL2 proxy troubleshooting note

[nightcityblade]

Summary

Fixes #1930

Adds a WSL2-specific proxy troubleshooting note showing how to use a dedicated Playwright-launched browser with BrowserConfig.proxy_config when managed browser startup hangs behind a required system proxy.

List of files changed and why

• docs/md_v2/advanced/proxy-security.md - Documents the WSL2 proxy workaround in the existing troubleshooting section.

How Has This Been Tested?

• Ran git diff --check.
• Reviewed the rendered Markdown source and Python snippet for correctness.

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have added/updated unit tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes