Quickstart page doesn't clarify that the Functions tab uses a deprecated API

Nobody has claimed this yet.

## Description
The Snaps quickstart page shows two code examples — JSX and Functions.
The "Functions" tab uses `panel()` and `text()` from `@metamask/snaps-sdk`,
which is the older deprecated approach. However, there is no note or warning
telling readers that the JSX approach is preferred and the Functions API
is legacy/deprecated.

## Page URL
https://docs.metamask.io/snaps/get-started/quickstart/

## Expected behavior
Add a deprecation note or warning on the "Functions" tab making it clear
that JSX is the recommended approach for new Snaps, and that the
Functions-based API is maintained for legacy purposes only.

## Why this matters
New developers copying the Functions example may use a deprecated pattern
without realizing it, leading to confusion later.

Open the Snaps quickstart page at https://docs.metamask.io/snaps/get-started/quickstart/ and inspect the Functions tab alongside the JSX example. Add a clear deprecation note stating that JSX is recommended for new Snaps and the Functions API is retained for legacy use. Done means readers can identify the Functions example as deprecated before copying it.

Written by the indexing model from the issue text.