docs: add Windows UTF-8 encoding note to getting started guide (#4699)

The `echo` command on Windows (both cmd.exe and PowerShell) may create
files with non-UTF-8 encoding, causing commitlint to fail reading the
configuration. Added a note advising Windows users to create the config
file manually or use PowerShell's Out-File with explicit UTF-8 encoding.

Closes #788

Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>