docs: migrate functions reference layout to headers

[l-zeuch]

This PR tracks the progress of migrating the layout of the functions reference from a table-based layout to a header-based layout.

I've pushed a preliminary commit showcasing how I imagine the layout going forward and am looking for feedback on the proposal.

~~As I continue working on this, I will amend this commit and subsequently force-push this branch.~~ I will push fixups as I go and then rebase + squash before merging.

Terms

• [x] I have read and understood this project's Contributing Guidelines