Pin RTD to sphinx 5.3 (#1673)

Sphinx handling of `html_logo` and `html_favicon` changed in version 6,
but changing to the documented logo_url and favicon_url still doesn't
work for some reason. But Sphinx 5.3 works, so let's pin to that. This
will need to get sorted out at some point, when there's a need to move
to a newer sphinx.

Signed-off-by: Cary Phillips <[email protected]>

website/requirements.txt

@@ -1,3 +1,3 @@
-sphinx >= 5.0
+sphinx == 5.3
 breathe
 sphinx-press-theme