[codex] Refocus docs on binding-specific guidance

[AdamEssenmacher]

Summary

This PR resets /docs around the principle that the native Firebase, Google, and ML Kit Apple SDK documentation is the source of truth for product behavior.

Changes include:

• Adds docs/README.md as the docs policy and index.
• Rewrites Firebase NuGet README content to cover only package IDs, namespaces, target frameworks, native-doc links, version alignment, and binding-specific caveats.
• Replaces copied Firebase and Google GettingStarted.md / Details.md walkthroughs with stable-path stubs that link to official native docs.
• Simplifies build, publishing, and shared package README docs around binding/package concerns.
• Adds a top-level README quick link to the docs index.

Why

These packages are thin .NET bindings over native Apple SDKs, so duplicated product walkthroughs drift quickly and can mislead users. Repository docs should describe only the binding and packaging details that native docs cannot cover.

Validation

• git diff --check -- docs Readme.md
• Local markdown link/anchor scan: 0 broken local links, 0 missing reference labels.
• Official Firebase/Google/GitHub docs URL check: 28 checked, 0 failed.

Notes

The PR intentionally excludes existing non-doc worktree changes related to Firebase 12.9.0 package/source/test updates.