Close #6446: Add sphinx.ext.durations to inspect durations of build

doc/usage/extensions/duration.rst

@@ -0,0 +1,11 @@
+:mod:`sphinx.ext.duration` -- Measure durations of Sphinx processing
+====================================================================
+
+.. module:: sphinx.ext.duration
+   :synopsis: Measure durations of Sphinx processing
+
+.. versionadded:: 2.3
+
+This extension measures durations of Sphinx processing and show its
+result at end of the build.  It is useful for inspecting what document
+is slowly built.

doc/usage/extensions/index.rst

@@ -23,6 +23,7 @@ These extensions are built in and can be activated by respective entries in the
    autosummary
    coverage
    doctest
+   duration
    extlinks
    githubpages
    graphviz