docs/develop: Mass directory restructure.

Now vaguely follows the tree structure of "src", with the exception of
directories that described subsystems spanning more than one "kit" or
"server" (e.g.

docs/develop: Mass directory restructure.

Now vaguely follows the tree structure of "src", with the exception of
directories that described subsystems spanning more than one "kit" or
"server" (e.g. "media", "midi", "bluetooth") -- these have been left as their
own top-level directory within docs/develop.

show more ...