Explicitly tell myst to accept internal links to multiple levels of headings

Some dumb change requires this to be explicit now.

conf.py

@@ -47,6 +47,8 @@ extensions = [
     'sphinx_rtd_theme',
 ]
 
+myst_heading_anchors = 5
+
 # Add any paths that contain templates here, relative to this directory.
 templates_path = ['_templates']