[ci skip] add "Qu" to Backends Features list
Merge pull request #17626 from y-yagi/queue_adapters_doc

[ci skip] add "Qu" to Backends Features list
Don't let #{application,framework,full}_trace be nil

Those three can be nil when exception backtrace is nil. This happens and
that forced a couple of nil guards in the code. I'm proposing to make
those always return an array, even on nil backtrace.
[CI SKIP] Missing space.
Merge pull request #17636 from gsamokovarov/exception-wrapper-less-nils

Don't let #{application,framework,full}_trace be nil
Merge pull request #17635 from tgxworld/missing_space

[ci skip] Missing space in Active Job guide for referring to Action Mailer
add Nokogiri to docs generation guides
Merge pull request #17637 from gregmolnar/docs

add Nokogiri to docs generation guides [ci skip]