Split top-level specification files subsections into multiple files

[kMutagene]

I suggest we split the current top-level .md files into files in a subfolder according to section headers.

Example: ARC specification.md should turn into a folder that contains each section as a page. The advantage of this is we can stop working on a single file and improve tracking of what PRs actually change, because they will then target a file in the folder structure instead of a monolithic markdown file.

Additionally, a folder structure like that nicely translates into a web version of the specs that does not overload a single page with hard to skim-through content.