doc.kak: Also search through plugins (stdlib and per-user) for docs.
doc.kak: Also search through plugins (stdlib and per-user) for docs.

This makes the somewhat-dubious assumption that every plugin will have uniquely-
named documentation files, instead of automatically putting every plugin's docs
into a namespace. However, plugins already have to deal with flat namespaces for
commands, options, filetypes, etc. so one more shouldn't hurt.

Fixes #2466.

alexherbo2/kakoune,casimir/kakoune,alexherbo2/kakoune,danr/kakoune,jkonecny12/kakoune,danr/kakoune,alexherbo2/kakoune,lenormf/kakoune,danr/kakoune,danr/kakoune,casimir/kakoune,Somasis/kakoune,casimir/kakoune,Somasis/kakoune,lenormf/kakoune,mawww/kakoune,jkonecny12/kakoune,casimir/kakoune,jkonecny12/kakoune,alexherbo2/kakoune,lenormf/kakoune,lenormf/kakoune,jkonecny12/kakoune,mawww/kakoune,mawww/kakoune,mawww/kakoune,Somasis/kakoune,Somasis/kakoune