Rename khoj config yml file to follow more specific khoj*.yml pattern

- That is, sample_config.yml is renamed to khoj_sample.yml
- This makes the application config filename less generic,
  more easily identifiable with the application
- Update docs, app accordingly

.gitignore

@@ -11,4 +11,4 @@ src/.data
 /build/
 /dist/
 /khoj_assistant.egg-info/
-/config/*.yml
+/config/khoj*.yml