Documentation: Add summary of what this add-on does and doesn't do

[thijzert]

This commit moves some general WireGuard promo copy to its own heading, and adds a short summary of the add-on capabilities to the top of the documentation page.

After several hours of debugging spanning multiple days, I found issue #254, which concluded that what I was trying is apparently not possible. This PR adds this result to the top of the documentation page.

Summary by CodeRabbit

• Documentation
  • Updated WireGuard Home Assistant Community Add-on documentation to highlight using the add-on as a bridge server for secure client device connections without exposing the Home Assistant interface.