Make article-only catalog pages language-less #240
Description
Currently, the language of pages in article-only catalogs, i.e., catalogs that define a @TechnologyRoot, is "Swift" (arbitrarily), since there isn't really a language that can be inferred for them.
We should instead track these as language-less (availableSourceLanguages: []) or as a special .undefined language. This will require relaxing the navigator index logic to still include these pages, despite not being of language Swift, Objective-C, or Data.
Swift-DocC Version Information
Swift-DocC version: e761687
rdar://118461894