Add PyCharm IDE specific .gitignore files. (#2876)

Add ```.idea/``` folder to ```.gitignore``` for devs. who work in
PyCharm IDE.

```python

# JetBrains PyCharm:
.idea/

```

.gitignore

@@ -22,3 +22,6 @@ backend/openapi.json
 .pytest_cache/
 
 /docker-compose.override.yml
+
+# JetBrains PyCharm:
+.idea/