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.comSigned-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/dev-tools/sparse.rst

@@ -9,6 +9,8 @@ Sparse is a semantic checker for C programs; it can be used to find a
 number of potential problems with kernel code.  See
 https://lwn.net/Articles/689907/ for an overview of sparse; this document
 contains some kernel-specific sparse information.
+More information on sparse, mainly about its internals, can be found in
+its official pages at https://sparse.docs.kernel.org.
 
 
 Using sparse for typechecking