docs: declare the brainpy / brainpy.state relationship

[chaoming0625]

Summary

• New docs/project/relationship.rst: an additive (non-deprecation) declaration of how classic brainpy and the state-based brainpy.state layer relate, modeled on Numba's CUDA status notice.
• docs/index.rst: short titled admonition on the landing page + project/relationship added to the Project toctree.

Validation

• sphinx -b dummy (notebook execution off): exit 0, 0 warnings; relationship.html builds and :doc: links resolve.

🤖 Generated with Claude Code

Summary by Sourcery

Document the relationship between classic brainpy and the state-based brainpy.state layer and surface this explanation in the main docs landing page.

Documentation:

• Add a dedicated documentation page describing how classic brainpy and brainpy.state relate and are intended to be used together.
• Update the docs index to highlight and link to the new relationship page from the project section.

[sourcery-ai]

Reviewer's guide (collapsed on small PRs)

Reviewer's Guide

Adds documentation clarifying how the classic brainpy API relates to the newer state-based brainpy.state layer, and surfaces this relationship on the docs landing page and in the project navigation.

File-Level Changes

Change	Details	Files
Introduce a dedicated documentation page describing the relationship between classic brainpy and brainpy.state.	Create new reStructuredText document outlining the status and intended use of classic brainpy vs brainpy.state. Model the relationship and status notice on Numba’s CUDA status-style explanation, including guidance and forward-looking notes. Include explicit, non-deprecating language to clarify that classic brainpy remains supported while positioning brainpy.state as the state-based layer.	docs/project/relationship.rst
Expose the new relationship documentation page in the main docs index and project navigation.	Add a short, titled admonition to the docs landing page pointing readers to the relationship documentation. Insert the new relationship page into the Project toctree so it appears in navigation and is reachable via :doc: references. Ensure Sphinx builds cleanly with the new page and links (dummy builder, no notebooks) with zero warnings.	docs/index.rst

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[github-actions]

🎉 Congratulations on opening your first pull request in brainpy.state! Thank you for your contribution!

A maintainer will review your changes soon. Please make sure:

• ✅ Your code follows our style guidelines
• ✅ All tests pass
• ✅ Documentation is updated (if applicable)
• ✅ You've filled out the PR template completely

We appreciate your effort in making brainpy.state better. If you have any questions, feel free to ask in the comments.

Welcome to the brainpy.state community! 🌟

[sourcery-ai]

Hey - I've reviewed your changes and they look great!

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