Fix visible \n characters in README's mermaid diagram

[Copilot]

The mermaid diagram in the README contained visible \n characters instead of proper line breaks, making the flowchart difficult to read. These literal \n characters appeared in three node labels within the decision tree diagram:

• id2B: "Do you want to add an unsupported package? \n either a package..."
• id2C: "Do you want to use Mariner for your project? \n ex. bare metal..."
• id3B: "Do you want to add a common package? \n either a package..."

Changes made:

• Replaced all literal \n characters with proper <br> tags in the mermaid flowchart
• Preserved all other diagram content and formatting
• No functional changes to the repository structure or build process

Result:
The mermaid diagram now renders with proper multi-line text formatting instead of showing raw \n characters, improving readability of the decision tree that helps users choose between the CBL-Mariner and CBL-MarinerTutorials repositories.

Fixes #71.

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