Add FAQ item about Qubes OS comparisons with secureblue

[EsseLowNitro]

How secureblue compares with Qubes OS is a topic brought up quite frequently in the secureblue Discord and other places where the project is discussed, and etching in the FAQ the explanation that these two projects have different goals is probably harmless and might be desirable. Self-assigned just in case, I can write the FAQ item today.

[martinoc000]

Any link? I didn't find it on the website.

[RoyalOughtness]

@martinoc000 This ticket is in an open state, meaning that it hasn't been implemented.