As of the 20.04 cycle, the Ubuntu serverguide is changing from DocBook to discourse markdown (markup?) language. A link will be added shortly to the main landing page at help.ubuntu.com, but meanwhile here is a direct link: https://ubuntu.com/server/docs. Every web page has a link to the discourse source page at the bottom, where users can make suggestions, point out errors, and (if you have rights) fix bugs on pages.
for 20.04 Ubuntu serverguide changing URL and markdown language
Archived thread 2431724 from Documentation and Community Wiki Discussions. Markdown source: Documentation_and_Community_Wiki_Discussions/thread_2431724_for_20.04_Ubuntu_serverguide_changing_URL_and_mark.md