docs: note exception-safe cleanup of JS Yoga trees

[greekr4]

Summary

The JavaScript docs already warn that nodes must be freed manually, but don't mention two things we ran into in production (a WASM-based canvas editor that builds a throwaway Yoga tree per layout pass):

1. Nodes live in the WebAssembly heap, so leaked nodes are invisible to the JS garbage collector and accumulate until Node.create() fails.
2. If a measure function (or any code between create and free) throws, the whole tree leaks unless freeing happens in a finally block.

This adds a short paragraph and a try/finally example to the existing warning in the JavaScript tab of "Laying out a Yoga tree".

Test Plan

Docs-only change. Verified the MDX renders (admonition + code fence) and the relative link to external-layout-systems.mdx resolves.