[mix_model]: Fix issues and improve the lecture

[bishmaybarik]

This PR fixes the following issues:

• Fixes [mix_model] FIX: links are full web links across the lecture #577

The issue was the following:

• style guide review
• fix full web links that are currently used across the lectures with a range of link text, doc links.

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[bishmaybarik]

hi @mmcky , I have made some changes and made sure I pushed everything to this branch. although I have checked it myself, it'll be nice to have it reviewed by you :-)

if there are further issues with the lecture, please let me know!

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[Copilot]

Pull Request Overview

This PR fixes formatting, style, and link issues in the mix_model lecture to align with style guidelines and improve readability. The changes primarily involve converting raw URLs to proper markdown links, fixing capitalization in headings, and correcting a few punctuation errors.

Key Changes

• Converted raw web links to proper markdown format with descriptive text
• Fixed capitalization in section headings to follow consistent style
• Made minor punctuation and text corrections