Improve docs consistency and add open_dataset reference

[jdbcode]

Summary

• add a new canonical open_dataset reference page for xr.open_dataset(..., engine='ee')
• improve docs consistency across README/quickstart/installation/concepts/guide/performance
• standardize endpoint and AOI wording
• reduce install-message duplication by pointing to canonical installation guidance
• de-emphasize legacy examples in rendered docs and add a status note
• add repository-level AGENTS.md with vendor-neutral guidance for automation tools and AI assistants
• improve backend docstring formatting in xee/ext.py so rendered parameter docs are clearer

Docs structure updates

• add docs/open_dataset.md and include it in docs navigation
• add docs/examples.md as an orphan page with a pre-v0.1.0 compatibility warning
• update links/references to avoid broken docs references and improve local-doc navigation

Validation

• pixi run -e docs docs-build

Notes

• examples are intentionally downplayed in rendered docs until they are updated for v0.1.0.