add FreeBSD to installation instructions

[oregonpillow]

Describe the bug

Couple suggestions:

1. Abit of a stylistic change, tell me if you dont like it: the paragraph about recommending to install latest version is repeated for Debian/Ubuntu systems. I think for readability you can combine Debian based systems within a single section. Paragraph in question:

mergerfs/mkdocs/docs/setup/installation.md

Lines 16 to 19 in 5e584f2

	Most Debian installs are of a stable branch and therefore do not have
	the most up to date software. While mergerfs is available via `apt` it
	is suggested that users install the most recent version available from
	the [releases page](https://github.com/trapexit/mergerfs/releases).

2. list the OS alphabetically

3. add FreeBSD installation - you might like this story...after recommending mergerfs to a colleague at work, I didn't realize he is one of the FreeBSD maintainers, a couple months go by and he informs me that he decided to port and maintain mergerfs for FreeBSD :) https://www.freshports.org/filesystems/mergerfs

[oregonpillow]

proposed changes would look like this: https://oregonpillow.github.io/setup/installation/

[trapexit]

I did not realize someone had patched mergerfs for FreeBSD. Some time ago I made some breaking changes and never got around to trying to fix them.

I would like if we could just get those changes upstreamed so I know what's going on. We can add back the FreeBSD build target for tests.

I separated Debian and Ubuntu because in the past there were newbies who didn't really understand they were more or less the same. And IIRC there might have been slight differences at one time. I'm not opposed to reworking them but that is the reason. A lot of mergerfs users are not terribly knowledgeable about Linux and filesystems so I end up having to add a lot of stuff to the docs that others may find unnecessary.

[trapexit]

The patches seem small enough to be incorporated without much issue. Seems like he just faked some commands so they are nops. It would be good to add the patches to mainline and change the docs to indicate that some behaviors will not work on freebsd. Also, there are other issues with freebsd that I used to have in the docs but no longer because when things broke I removed them. But for example... freebsd can't have per thread identity like Linux so threading is limited due to a contention on locks.

[oregonpillow]

So we can hold off on adding FreeBSD as installation option, until you've added it back as build target? If I understand correctly.

As for the Debian/Ubuntu, I did suspect that might have been the reason you explicitly listed it. I can definitely revert those changes, or keep them. I think it's pretty clear still to inexperienced users but it's your call.

[trapexit]

I just did some work pulling in some of the patches and will push a PR later with that and some updated docs. It's only for FreeBSD 14 right now. I'll load up 13 later and see what else might be needed.

I'm fine with the Debian based os compression but let me push my freebsd changes first and then apply to that.

[trapexit]

Check out the latest in master. #1398 I've added FreeBSD support and added freebsd relevant parts to the docs. Feel free to clean up the Install section for Debian.