[Docs] Add missing lessons 25-27 to sidebar navigation

[GziXnine]

Description

Adds missing lesson entries to the documentation sidebar (docs/_sidebar.md). The sidebar was missing the three newest lessons, making them inaccessible via Docsify navigation.

Changes

Added sidebar entries for:

• Lesson 25: Code Editor (8-code-editor/1-using-a-code-editor/)
• Lesson 26: Chat Project (9-chat-project/)
• Lesson 27: AI Framework Project (10-ai-framework-project/)

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

Testing / Verification

• Verified all paths exist in the repository
• Follows existing sidebar formatting conventions
• Sequential lesson numbering maintained (25, 26, 27)

Impact

Users can now navigate to all lessons via the Docsify documentation sidebar.