As suggested open openstack-discuss ML[1], some sphinxcontrib packages
have not been updated for several years and might going to be
maintained anymore. In tacker-specs repo, many diagrams are compiled
with sphinxcontrib-seqdiag and sphinxcontrib-nwdiag. This update is to
drop using the packages and add image files instead. The embedded
source codes are remained as separated files and named as "*.diag".
In addition, it includes two updates other than that.
* usage of the dropped diagram support described in the
`specs/template.rst` is also removed because it's no longer
supported.
* Upgrade the version of `pillow` to the latest 11.0.0 since
installation is failed if the version is old.
[1] https://lists.openstack.org/archives/list/openstack-discuss@lists.openstack.org/message/4IID4UEXY4PJJGBTMFMTRYLKJIN4GOQ6/
Change-Id: I8cede6de0770b68a9984617643aa4aa81e47ba5c
af542c4418