Rework FreeBSD start page for newcomer

- Additional instructions to build
- Remove duplicate info about OpenZFS

Co-authored-by: Ryan Moeller <ryan@freqlabs.com>
Co-authored-by: George Melikov <mail@gmelikov.ru>
Signed-off-by: George Melikov <mail@gmelikov.ru>

Apply suggestions from @freqlabs

Co-authored-by: Ryan Moeller <ryan@freqlabs.com>

docs/Getting Started/index.rst

@@ -14,7 +14,7 @@ documentation <https://pthree.org/2012/04/17/install-zfs-on-debian-gnulinux/>`__
    ArchLinux <https://wiki.archlinux.org/index.php/ZFS>
    Debian/index
    Fedora
-   FreeBSD <https://zfsonfreebsd.github.io/ZoF/>
+   FreeBSD
    Gentoo <https://wiki.gentoo.org/wiki/ZFS>
    NixOS <https://nixos.wiki/wiki/NixOS_on_ZFS>
    openSUSE <https://software.opensuse.org/package/zfs>