docs: explain XR render target pattern

[buyicoder]

Summary

• Add an advanced docs page explaining how to render to a WebGLRenderTarget while an XR session is active.
• Document the save/restore pattern for render target, viewport, and XR flags.
• Link the new page from the getting started tutorial list.

Test plan

• git diff --check

Closes #373

[buyicoder]

Thanks for the docs issue. I kept this focused on the practical render target pattern that the XR layer code already uses internally: save the current render state, temporarily disable XR for the offscreen pass, then restore the render target, viewport, and XR flags.

I only ran git diff --check locally since this is docs-only.