docs: create reproducible html

docs: create reproducible html

The Makefile uses wildcard to collect a list of files. The resulting
list of files is in directory order, which is random. As a result the
generated html files will differ when build on different hosts.

Use the built-in sort function to get a stable list of files.

Signed-off-by: Olaf Hering <olaf@aepfle.de>
Cc: Ian Jackson <ian.jackson@eu.citrix.com>
Cc: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Cc: Ian Campbell <ian.campbell@citrix.com>
Cc: Wei Liu <wei.liu2@citrix.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>