docs: add Homebrew tap scaffold

[0thomasholland]

Description

Adds a versioned RapidRAW Cask and publishing instructions for a maintainer-owned CyberTimon/homebrew-rapidraw tap. This provides an interim Homebrew installation channel while signed and notarized macOS releases remain the long-term goal.

Type of Change

• Documentation update

Changes Made

• Add Cask definition for the current macOS ARM and Intel release assets.
• Document creation and maintenance of an upstream Homebrew tap.
• Document the user-facing Gatekeeper limitation until notarization is enabled.

Testing

• Validated Cask Ruby syntax.

Checklist

• My code follows the project's code style.
• My changes generate no new warnings or errors in syntax validation.

Additional Notes

Maintainers retain ownership of the tap repository and release-update process.

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