Add initial doc for future R2R platform-native image (Mach-O, composite only)

[elinor-fung]

Create a doc with a tentative plan for the platform-native R2R support we are intending to add (Mach-O, composite only). We can update this as we go.

[Copilot]

Pull Request Overview

This PR adds initial documentation for planned ReadyToRun platform-native image support in .NET 11, specifically for Mach-O format on macOS. The documentation outlines the high-level design for supporting composite ReadyToRun images using platform-native formats instead of the current PE format used across all platforms.

Key changes:

• Creates a new design document detailing the plan for Mach-O support in composite ReadyToRun scenarios
• Updates existing ReadyToRun documentation to reference the new platform-native envelope support
• Introduces new crossgen2 flags and runtime callback mechanisms for handling platform-native R2R images

Reviewed Changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 1 comment.

File	Description
docs/design/coreclr/botr/readytorun-platform-native-envelope.md	New design document outlining the plan for platform-native R2R support with Mach-O format
docs/design/coreclr/botr/readytorun-overview.md	Updates overview to reference future platform-native envelope support and current state
docs/design/coreclr/botr/readytorun-format.md	Updates format documentation to link to the new platform-native envelope design

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[elinor-fung]

/ba-g markdown only