website/contact: Remove GitHub advice
The doc project has demonstrated a consistent and united consensus that
we do not want to receive patches on GitHub. Remove the suggestion to
send patches on the GitHub silo that only carlavilla and I will see.
Event: EuroBSDcon 2026
Reviewed by: carlavilla, wosch
Suggested by: wosch
Differential Revision: https://reviews.freebsd.org/D59617