Add public doc FAQ for common points of confusion due to current limitations

[anngvu]

We have user feedback like:

I was confused because I looked at the library prep column and saw RNA-seq resources

from NFOSI-774

Currently, we can't conditionally show a subset of the valid values depending on the template (though this might be doable eventually with schematic with "context" feature). For example, to have different libraryPrep options for RNA vs WGS, we'll have to have different attributes called RNALibraryPrep or WGSLibraryPrep that are linked to different enums, but we can't do this because the key should just be "libraryPrep" when submitted. (In fact, an alternative approach was suggested for "submit as", as another workaround to contexts if it's too hard to implement, see internal Slack.)