Add a highly visible ASCII-art warning banner that is printed at
startup when the configured IP prefixes fall outside the standard
Tailscale CGNAT (100.64.0.0/10) or ULA (fd7a:115c:a1e0::/48) ranges.
The warning fires once even if both v4 and v6 are non-standard, and
the warnBanner() function is reusable for other critical configuration
warnings in the future.
Also updates config-example.yaml to clarify that subsets of the
default ranges are fine, but ranges outside CGNAT/ULA are not.
Closes#3055
5105033224