Add sidebar toggle to landing page

[melissawm]

Description

Adds a sidebar toggle to the landing page, so that the main docs menu is also optionally visible there.

Tests

Local docs build looks like this, when the sidebar is toggled:

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.