Add .readthedocs.yml config for installing breathe dependency

.readthedocs.yml

@@ -0,0 +1,2 @@
+conda:
+    file: docs/environment.yml

docs/environment.yml

@@ -0,0 +1,7 @@
+name: nuklear-docs
+
+channels:
+    - conda-forge
+
+dependencies:
+    - breathe