[Docs] Clarify Next.js version support in Amplify Gen 2

[jaouad-ankour]

Describe the content issue:
The documentation contains conflicting information about Next.js version support that may confuse users:

1. Quickstart links to Next.js 16 documentation: The bullet point "If you are new to these technologies, we recommend you go through the official React, Next.js, and TypeScript tutorials first" links to Next.js 16.0.1 documentation, which may imply that Next.js 16 is supported.
2. Hosting documentation states different version range: The AWS Amplify Hosting documentation explicitly mentions support for Next.js versions 12 through 15 only.
3. npm package confirms Next.js 16 is not supported: The @aws-amplify/adapter-nextjs package shows peer dependency range >=13.5.0 <16.0.0, confirming Next.js 16 is not yet officially supported.

Suggested fix:

Add a note specifying which Next.js versions are currently supported by AWS Amplify (e.g., "AWS Amplify currently supports Next.js versions 1X. through 15.x")