Update lecture markdown files

[Zahra-khanzadeh]

Summary Changes

Functions lecture

• Improved wording and sentence structure for better readability.
• Fixed parentheses-related issues to improve function readability in RTL (right-to-left) layout.

Getting Started lecture

• Updated the text related to icon placement to align with the site theme.
• Improved wording and terminology for better readability.

Python Essentials lecture

• Translation refined for clarity and better readability.
• Added to the TOC (lectures/toc.yml) to ensure proper navigation in the course structure.

Overall
All lectures are now fully aligned with the English version.

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