The Json and Yaml and Style pages should have links on Language specification (hopefully it's not a caching issue that I can't see them there!...). [also, remove defunct "incubation"].
Also Preprocessing#Preprocessing Json has a 404 link to https://plantuml.com/preprocessing-json (it should probably be http://wiki.plantuml.net/site/preprocessing-json (this page should also have a link from plantuml.com/json)).
[I've edited this now I realise that Json and Yaml are linked to from "Home" (not a page I visit very often due to my bookmarks). But I still think they should be in the language specification sitemap page - which is the page I use most for navigation.]
Sorry, I see I've partially duplicated https://forum.plantuml.net/12537/no-link-to-json-doc-from-language-specification-page?show=12537#q12537