Make it easier for LLMs to navigate through xcube

[forman]

Is your feature request related to a problem? Please describe.

Improve responses of LLMs regarding xcube by making it easier for LLMs to navigate the xcube code, docs, examples, tests, and key concepts.

How this helps

• Focus: guides LLMs to only the important parts of the repo.
• Context first: places core docs & high-level concepts at the top.
• Ignore noise: excludes configs/CI files you usually don’t want to summarize.

Describe the solution you'd like

Add a llms.txt file into the root. See llms.txt Hub