Documentation: Update details of The Linux Kernel Module Programming Guide

Recently, the content and examples of the book "The Linux Kernel Module
Programming Guide" are being actively maintained and added on Github[1].
Currently, the book is being regularly built into webpage and pdf
file using Github static page[2].

[1]: https://github.com/sysprog21/lkmpg
[2]: https://sysprog21.github.io/lkmpg/



Signed-off-by: Chun-Hung Tseng <henrybear327@gmail.com>
Link: https://lore.kernel.org/r/20210820222152.971174-1-henrybear327@gmail.com


[jc: fixed docs-build warnings]
Signed-off-by: Jonathan Corbet <corbet@lwn.net>