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 <[e-mail]>
Reviewed-by: Carlos Bilbao <[e-mail]>
Signed-off-by: Jonathan Corbet <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]

Documentation/process/kernel-docs.rst

···
 Published books
 ---------------
 
+    * Title: **The Linux Memory Manager**
+
+      :Author: Lorenzo Stoakes
+      :Publisher: No Starch Press
+      :Date: February 2025
+      :Pages: 1300
+      :ISBN: 978-1718504462
+      :Notes: Memory management. Full draft available as early access for
+              pre-order, full release scheduled for Fall 2025. See
+              https://nostarch.com/linux-memory-manager for further info.
+
     * Title: **Practical Linux System Administration: A Guide to Installation, Configuration, and Management, 1st Edition**
 
       :Author: Kenneth Hess