Docs: Add Know-how section to home page Documentation Sections

[Copilot]

The home page Documentation Sections was missing information about the Know-how section, which contains practical system administration guides that are part of the documentation navigation.

This PR adds a description of the Know-how section to the Documentation Sections in docs/index.md, positioned logically between the Customization and Reference sections. The new content:

• Introduces the Know-how section as "practical system administration guides that complement your Zephyr workflow"
• Provides direct links to all three know-how guides: creating bootable USB drives, securely erasing disks, and system rescue operations
• Maintains consistency with the existing documentation style and format

The change is minimal and surgical - only adding the missing information while preserving all existing content and structure. The documentation builds successfully with MkDocs and all links work correctly.

Fixes #213.

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