Add or doc support for making `/var` transient

We should really support this; I think one could probably hack it up with a post-switchroot systemd unit that just mounts a tmpfs for `/var`, but I suspect at least some people *will* want to spool to the real filesystem for large data and not be limited by RAM (ok modulo enabling file-backed swap on the real root, which we should also definitely add support for...yeah see below).

Anyways, it'd be easy to add `var.transient` to go alongside our existing `etc.transient` and `root.transient`.

## Tangent: automatic swap file support

Swap partitions are well known and supported. However, there's also swap files because basically dealing with partitions stinks - especially resizing.

It might be nice if we supported something like... `/sysroot/autoswap` or `/sysroot/bootc/swap` that we detect in the initramfs and if present we enable by default. Although we'd also need to expose a nice declarative way to initialize that at install time as embedded in a container, probably in the install config? Hmm though there's no reason not to support changing it "day 2". In the general case of course one can just do this manually in the initramfs, but customizing the initramfs in this way has a higher bar and is more likely to break. I could imagine this being a default part of dracut...something like a rd.swap= kernel arg.