docs: Adding swap recommends fallocate, however fallocate has holes

[Eaterminer]

Creating swap with holes is generally regarded as a really bad idea.
dd if=/dev/zero of=PATH is a better method because it doesn't create holes.

[tormath1]

Hello @Eaterminer and thanks for the heads-up. I think a clean way to create swap files would be to rely on mkswap directly. Would you be happy to contribute to the documentation here: https://github.com/flatcar/flatcar-website/blob/faf23bd5283524e180cb83a8e0c1cc268c10b574/content/docs/latest/setup/storage/adding-swap.md?plain=1#L67-L69 and update those lines to something like:

mkswap -U clear --size 1024m --file /var/vm/swapfile1

[Eaterminer]

Sure, I'll get a PR by the end of today :)

[Eaterminer]

@tormath1 PR opened, #381