install.html: Update gentoo status

Might also be interesting to link to https://repology.org/project/woob/versions so it works for all distributions.

Note: GURU is similar to Archlinux AUR but with light code reviews by trusted contributors before deployment.

If you want to check the package:

• Direct source: https://gitweb.gentoo.org/repo/proj/guru.git/tree/www-client/woob
• Community-provided interface: http://gpo.zugaina.org/www-client/woob