Add sdscoq7snqtznauu.onion to torproject.org/docs/debian.html

deb.torproject.org is now also served via THS: http://sdscoq7snqtznauu.onion/ See #17937 (moved) and https://onion.torproject.org/

Please also add that to https://www.torproject.org/docs/debian.html.en

Trac:
Username: ilf

To upload designs, you'll need to enable LFS and have an admin enable hashed storage. More information