Breathe needs to be initialized in parent sphinx project (#606)

Author: pyohannes

docs/public/conf.py

@@ -44,8 +44,15 @@
 # extensions coming with Sphinx (named 'sphinx.ext.*') or your custom
 # ones.
 extensions = [
+    "breathe",
 ]
 
+breathe_projects = {
+        "OpenTelemetry C++ API": "../../api/docs/doxyoutput/xml"
+}
+
+breathe_default_project = "OpenTelemetry C++ API"
+
 primary_domain = "cpp"
 
 higlight_language = "cpp"