Update llms.txt

Update sites/docs/src/content/llms.txt, a curated resource list that helps LLMs understand the Flutter framework.

Always start by reading the existing sites/docs/src/content/llms.txt to understand the current structure and content before making changes.

Actions

Based on the user-provided action, perform one of the following tasks:

Add a section

When asked to add a new section:

1. If not provided, ask for the section title and links.
2. Place the new section in a logical location relative to existing sections.