Add Prism tool references for image customization

[Copilot]

This PR adds references to the Prism tool (https://github.com/microsoft/azure-linux-image-tools) as the recommended way to build custom Azure Linux images.

Changes Made:

• Added a prominent note at the top of docs/building/building.md informing users about the Prism tool
• Added a similar note at the top of docs/packages/working_with_packages.md
• Added a reference in the main README.md for better visibility

All notes include:

• Clear indication that Prism is the recommended method for image customization
• Links to both the GitHub repository and the Prism Image Customizer documentation
• Clarification that the existing instructions are still valid but represent the legacy approach

These minimal changes ensure users are aware of the preferred method for building custom images while preserving all existing content and functionality.

Fixes #69.

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[dmcilvaney]

Can you double check if there are more image references? I can see default_images.md at least.

[Copilot]

Can you double check if there are more image references? I can see default_images.md at least.

I've added the Prism tool reference to default_images.md as suggested. The update follows the same format as the previous changes, highlighting Prism as the recommended method for building custom images. Changes are in commit 96921e0.

[dmcilvaney]

This file also talks about custom spec files, can you add a visible callout to look there if you are doing that?

[Copilot]

I've added a reference to the custom spec files section in the note at the top of the working_with_packages.md file. The updated note now directs users to the "Tutorial: Customize your Image with Unsupported Packages" section when they're creating custom spec files. Changes are in commit 6c3c2d3.