docs-rst: convert lsm from DocBook to ReST

This file is outdated. Still, as it is the only one left at
DocBook dir, convert it, and store it, with a .txt extension,
under Documentation/lsm.txt.

This way, we can get rid of DocBook from the building system,
without needing to wait for someone to take care of it.

Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>