Fix URLs in the development docs

* Fix dead links where possible
* Use online instead of local links to the BeBook

Change-Id: I250117dcccc0026925c56545cca7e3b4467f2c78
Reviewed-on: https://review.haiku-os.org/c/811
Reviewed-by: Adrien Destugues <pulkomandy@gmail.com>

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.