Add documentation on qubes-fwupdmgr usage

[rocodes]

Refs https://github.com/QubesOS/qubes-issues/issues/8813

Add how-to documentation on using qubes-fwupdmgr (and a cursory mention of fwupdmgr in other qubes).

Putting in draft mode because after I wrote this up, I came across https://github.com/QubesOS/qubes-doc/pull/1414, which touches more on why to update and more information about firmware/microcode and has less how-to, so maybe we could combine them.