docs/livepatch: Unify style of livepatch documentation in the ReST format

Make the structure of "Livepatch module Elf format" document similar
to the main "Livepatch" document.

Also make the structure of "(Un)patching Callbacks" document similar
to the "Shadow Variables" document.

It fixes the most visible inconsistencies of the documentation
generated from the ReST format.

Signed-off-by: Petr Mladek <pmladek@suse.com>
Acked-by: Joe Lawrence <joe.lawrence@redhat.com>
Acked-by: Josh Poimboeuf <jpoimboe@redhat.com>
Acked-by: Miroslav Benes <mbenes@suse.cz>
Reviewed-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>
Reviewed-by: Kamalesh Babulal <kamalesh@linux.vnet.ibm.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>