Create release notes for ReadTheDocs

[bvandermoon]

Description

Create a new release notes page for ReadTheDocs. This page will be used to document notable changes for every MaxText release.

Since the first release includes all of MaxText up to September 2025, we start with an intro to MaxText instead of calling out new features, functionality, breaking changes, bug fixes, etc.

Tests

Doc creation only

Checklist

Before submitting this PR, please make sure (put X in square brackets):

• I have performed a self-review of my code. For an optional AI review, add the gemini-review label.
• I have necessary comments in my code, particularly in hard-to-understand areas.
• I have run end-to-end tests tests and provided workload links above if applicable.
• I have made or will make corresponding changes to the doc if needed, including adding new documentation pages to the relevant Table of Contents (toctree directive) as explained in our documentation.