Document graphviz requirement for sphinx.ext.graphviz (#14487)

Co-authored-by: Bénédikt Tran <10796600+picnixz@users.noreply.github.com>

AUTHORS.rst

@@ -118,6 +118,7 @@ Contributors
 * Thomas Waldmann -- apidoc module fixes
 * Till Hoffmann -- doctest option to exit after first failed test
 * Tim Hoffmann -- theme improvements
+* Tim Pillinger -- documentation improvements
 * Valentin Heinisch -- warning types improvement
 * Victor Wheeler -- documentation improvements
 * Vince Salvino -- JavaScript search improvements

doc/usage/extensions/graphviz.rst

@@ -14,6 +14,11 @@
 This extension allows you to embed `Graphviz <https://graphviz.org/>`_ graphs in
 your documents.
 
+.. important::
+
+   Graphviz must be present on your machine; please refer to the `official
+   instructions <https://graphviz.org/download/>`_ if it is not the case.
+
 It adds these directives:
 
 .. rst:directive:: graphviz