doc: add link to sparse's home page/internal docs

doc: add link to sparse's home page/internal docs

Sparse's home page used to be a wiki (sparse.wiki.kernel.org)
but this wiki only contained a short intro and the release notes.
But nowadays, sparse's main page is sparse.docs.kernel.org,
which contains all what was in the wiki but also other documentation,
mainly oriented about sparse's internals.

So, add a link to this in the kernel documentation.

Signed-off-by: Luc Van Oostenryck <luc.vanoostenryck@gmail.com>
Acked-by: Mike Rapoport <rppt@linux.ibm.com>
Link: https://lore.kernel.org/r/20200629161310.89783-1-luc.vanoostenryck@gmail.com
Signed-off-by: Jonathan Corbet <corbet@lwn.net>