Incorporate MSC2209 (notifications auth rules)

MSC: https://github.com/matrix-org/matrix-doc/pull/2209

The changes are slightly difficult to word without dumping the text in and playing a game of spot the difference, so we now use our pre-existing pygments support to render a representation of the difference. The difference is shown in markdown-like format instead of RST for ease of understanding. It's also not rendered HTML for largely complexity reasons.

scripts/gendoc.py

@@ -273,6 +273,7 @@ def rst2html(i, o, stylesheets):
                 writer_name="html",
                 settings_overrides={
                     "stylesheet_path": stylesheets,
+                    "syntax_highlight": "short",
                 },
             )