Add Native SDK philosophy guide

[henryoman]

What changed

• Added the official Native SDK introduction as Philosophy near the top of the Official section.
• Summarized its coverage of native rendering, explicit state, authoring, customization, and agent workflows.

Why

The introduction now documents the design principles behind Native SDK and belongs alongside the source repository and quick-start documentation.

Validation

• Confirmed https://native-sdk.dev/introduction is publicly accessible.
• Confirmed the branch changes only README.md with one added line.
• Confirmed the entry follows the repository's one-line description format and alphabetical order.

Ownership

This updates an official Native SDK documentation link; it does not submit a project owned by the contributor.