docs: Use skiplist when checking sysctl admin-guide

docs: Use skiplist when checking sysctl admin-guide

Use a skiplist to "skip" the titles in the guide documentation
(Documentation/admin-guide/sysctl/*) that are not sysctls. This will
give a more accurate account of what sysctl are miss-documented.

Signed-off-by: Joel Granados <joel.granados@kernel.org>