doc: kernel: Re-organise memory management API pages

The kernel documentation listed the memory management API pages under
two different sections: 'Kernel/Kernel Services' and
'Kernel/Memory Management' -- this creates unnecessary confusion and
makes it hard to look up all supported kernel memory management APIs.

This commit relocates the memory management API pages under
'Kernel/Kernel Services' to 'Kernel/Memory Management' so that all
memory management APIs provided by the kernel are described in one
unified section.

The link to the 'Kernel/Memory Management' index page is still left in
the 'Kernel Services' page because it may still be helpful to look at
it as part of the services provided by the Zephyr kernel -- it is just
more substantial than the rest and deserves more visibility.

Signed-off-by: Stephanos Ioannidis <root@stephanos.io>