build(ReadTheDocs): Set the primary domain in conf.py to C++

CRSF for Arduino is written in C++

docs/source/conf.py

@@ -3,7 +3,7 @@
 # -- Path setup
 import os
 import sys
-sys.path.insert(0, os.path.abspath('docs'))
+sys.path.insert(0, os.path.abspath('.'))
 
 # -- Project information
 
@@ -30,6 +30,8 @@
 }
 intersphinx_disabled_domains = ['std']
 
+primary_domain = 'cpp'
+
 templates_path = ['_templates']
 
 # -- Options for HTML output