feat(data): add Gentoo Linux installation instructions

[datbtw]

This PR adds Gentoo Linux installation instructions to the interactive installer via the official GURU overlay (app-i18n/fcitx5-lotus).

[datbtw]

Thanks for merging! One small addition that might be worth including —
uninstall instructions for Gentoo:

sudo emerge --deselect app-i18n/fcitx5-lotus
sudo emerge --ask --depclean

Warning: Always carefully review the package list before confirming [Yes].
Unlike other distros, Portage's --depclean re-evaluates the entire system
dependency graph against the @world set, so it may flag unrelated packages
if your system set isn't fully synchronized. If in doubt about any entry,
it's safer to answer [No] and leave it installed than to risk removing
something still in use.
Happy to submit a quick follow-up PR for this if you'd prefer!