admin, doc: introduce sphinxcontrib.seqdiag

for rendering sequence-diagram. unlike ditaa, seqdiag allows us to
create sequence-diagram without worrying about the layout. and the
syntax is quite like that of dot.

Signed-off-by: Kefu Chai <kchai@redhat.com>

admin/doc-requirements.txt

@@ -12,3 +12,4 @@ sphinx_rtd_theme == 0.5.2
 Sphinx-Substitution-Extensions
 typed-ast
 sphinxcontrib-openapi
+sphinxcontrib-seqdiag

doc/conf.py

@@ -121,7 +121,8 @@ extensions = [
     'ceph_commands',
     'ceph_releases',
     'ceph_confval',
-    'sphinxcontrib.openapi'
+    'sphinxcontrib.openapi',
+    'sphinxcontrib.seqdiag',
     ]
 
 ditaa = shutil.which("ditaa")
@@ -227,6 +228,10 @@ for c in pybinds:
 openapi_logger = sphinx.util.logging.getLogger('sphinxcontrib.openapi.openapi30')
 openapi_logger.setLevel(logging.WARNING)
 
+# seqdiag
+seqdiag_antialias = True
+seqdiag_html_image_format = 'SVG'
+
 # ceph_confval
 ceph_confval_imports = glob.glob(os.path.join(top_level,
                                               'src/common/options',