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

Readers looking for user-oriented information may benefit from it.
Signed-off-by: NJonathan Corbet <corbet@lwn.net>
Reviewed-by: NDavid Vernet <void@manifault.com>
Acked-by: NRandy Dunlap <rdunlap@infradead.org>
Link: https://lore.kernel.org/r/20220927160559.97154-8-corbet@lwn.netSigned-off-by: NJonathan Corbet <corbet@lwn.net>

Documentation/index.rst

@@ -76,6 +76,8 @@ developers seeking information on the kernel's user-space APIs.
    User-space tools <tools/index>
    userspace-api/index
 
+See also: the `Linux man pages <https://www.kernel.org/doc/man-pages/>`_,
+which are kept separately from the kernel's own documentation.
 
 Firmware-related documentation
 ------------------------------