docs: simplify conf.py and fix compatibility with upcoming Sphinx 6.0

Change-Id: I52415e37bcf237599aa5c4fa66605c34bc015626
diff --git a/docs/INSTALL.rst b/docs/INSTALL.rst
index f9492db..9f0f02b 100644
--- a/docs/INSTALL.rst
+++ b/docs/INSTALL.rst
@@ -85,7 +85,7 @@
 
 - doxygen
 - graphviz
-- sphinx >= 1.3
+- sphinx >= 4.0
 - sphinxcontrib-doxylink
 
 The following lists the steps to install these prerequisites on various common platforms.