[doc] Use native numfig extension

doc/conf.py

@@ -57,7 +57,6 @@ extensions = [
     'sphinx.ext.napoleon',
     'sphinx.ext.viewcode',
     'sphinxcontrib.mscgen',
-    'sphinx_numfig',
     ]
 
 # Always includes todos
@@ -66,6 +65,9 @@ todo_include_todos = True
 # Generates auto-summary automatically
 autosummary_generate = True
 
+# Create numbers on figures with captions
+numfig = True
+
 # If we are on OSX, the 'dvipng' path maybe different
 dvipng_osx = '/opt/local/libexec/texlive/binaries/dvipng'
 if os.path.exists(dvipng_osx): pngmath_dvipng = dvipng_osx