docs: add a man-pages link to the front page

Readers looking for user-oriented information may benefit from it.

Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Reviewed-by: David Vernet <void@manifault.com>
Acked-by: Randy Dunlap <rdunlap@infradead.org>
Link: https://lore.kernel.org/r/20220927160559.97154-8-corbet@lwn.net
Signed-off-by: Jonathan Corbet <corbet@lwn.net>