Documentation/kernel-docs.txt: adjust LDD references

- remove LDD versions 1 and 2, as there's already an entry for
  LDD3;
- add a link between LDD online and published entries.

Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>