doc: Convert the debugobjects DocBook template to sphinx

A couple of the most minor heading tweaks, otherwise no changes to the text
itself beyond the mechanical conversion.

Note that the inclusion of the kerneldoc comments from the source has never
worked, since exported symbols were asked for and none of those functions
are exported to modules.  It doesn't work here either :)

Cc: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>