Add Persian translation of functions lecture

- Translate functions.md from English to Persian
- Update _toc.yml to include functions lecture
- Maintain all code examples and technical syntax
- Preserve right-to-left (RTL) formatting for Persian text

Author: mmcky

lectures/_toc.yml

@@ -7,7 +7,7 @@ parts:
   - file: about_py
   - file: getting_started
   - file: python_by_example
-  # - file: functions
+  - file: functions
   # - file: python_essentials
   # - file: oop_intro
   # - file: names