See
https://usp.technology/specification for an example of how the Broadband Forum has recently been publishing some specs. This output is generated via an html-multi-writer.lua custom writer that contains the logic for splitting the output into multiple files (based on header level and/or specific classes). The writer also takes care of generating the navigation bar that you see at the top and bottom of the pages. The multiple files are then passed through pandoc again (to add the overall sidebar ToC and the top/bottom navigation bars, and to generate multiple standalone docs).
Please let me know if you'd like more details (the overall setup is not very easily shareable, because it's heavily dependent on rather complicated make rules, but I could probably share the custom writer and give some example command lines.)