docs etc.: https: Fix references to other Xen pages

docs etc.: https: Fix references to other Xen pages

Change the url scheme to https.  This is all in-tree references to
xenbits and the main website except for those in Config.mk.

We leave Config.mk alone for now because those urls are used by CI
systems and we need to check that nothing breaks when we change the
download method.

Signed-off-by: Ian Jackson <ian.jackson@eu.citrix.com>
Reviewed-by: Julien Grall <jgrall@amazon.com>