add install instructions for the *BSDs to the downloads page

[Izder456]

.

Not super familiar with the static generation software y'all use. But hopefully this is correct?

Thanks for reviewing this PR.

Cleanup commits are welcome on this PR.

[sfan5]

Sounds okay but maybe we should merge both BSDs and shorten it into a list, e.g.

• FreeBSD: install using pkg install luanti or compile from ports /usr/ports/games/luanti
• OpenBSD: install using pkg_add luanti or compile from ports /usr/ports/games/luanti

[Izder456]

Sounds okay but maybe we should merge both BSDs and shorten it into a list, e.g.

* FreeBSD: install using `pkg install luanti` or compile from ports `/usr/ports/games/luanti`

* OpenBSD: install using `pkg_add luanti` or compile from ports `/usr/ports/games/luanti`

Looking at one of NetBSD's package browsers, pkgsrc.se, I see there's a package for that too. So I added it. Having this as a list makes more sense IMHO. See my latest commit, is that good? @sfan5