Update to modern shpinx

The doc generations used a function removed since shpinx 4.0.
This replace it with the modern equivalent.

Signed-off-by: Yoan Picchi <yoan.picchi@arm.com>

Author: ypicchi-arm

doc/dev-reference/conf.py.in

@@ -23,7 +23,7 @@ import os
 # -- General configuration ------------------------------------------------
 
 # If your documentation needs a minimal Sphinx version, state it here.
-#needs_sphinx = '1.0'
+needs_sphinx = '4.0'
 
 # Add any Sphinx extension module names here, as strings. They can be
 # extensions coming with Sphinx (named 'sphinx.ext.*') or your custom
@@ -272,4 +272,4 @@ breathe_domain_by_extension = {"h" : "c"}
 # -- Add some customisation -----------------------------------------------
 
 def setup(app):
-    app.add_stylesheet("hyperscan.css") # Custom stylesheet for e.g. :regex:
+    app.add_css_file("hyperscan.css") # Custom stylesheet for e.g. :regex: