kernel-docs: Add book to process/kernel-docs.rst

kernel-docs: Add book to process/kernel-docs.rst

Add a reference to my new book, The Linux Memory Manager, an in-depth
exploration of the memory management subsystem, to
process/kernel-docs.rst.

This is not yet published, but the full draft is available on pre-order, so
it seems worthwhile adding it here. The situation is made clear in the
'notes' section.

The 'pre-release' was made available in February 2025, and full release is
scheduled for Fall 2025. The book's ISBN-13 is 978-1718504462.

The document will be updated upon release to reflect this.

Signed-off-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
Reviewed-by: Carlos Bilbao <carlos.bilbao@kernel.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Link: https://lore.kernel.org/r/20250218154303.45595-1-lorenzo.stoakes@oracle.com