Feature request: Support generating multiple formats

[ghost]

Instead of focusing on DocBook vs CommonMark, I think it would be good to instead introduce a --format param or something similar that allows outputing to a chosen format. This would allow nixdoc to be more versatile and flexible for future formats.

IMO, it would be good to generate something that other, fantastic, documentation generators might be able to use. doxygen, rustdoc, javadoc, sphinx etc. all generate documentation that is searchable, has links back to the source, have syntax highlighting, etc. Being able to output a format that they (or another tool) might understand would probably significantly decrease the effort needed to recreate output similar to theirs.

[infinisil]

Related: #129