docs: link brainpy.state relationship and fix stale doc URLs

[chaoming0625]

Summary

• docs/api.rst: add a brainpy.state relationship admonition; fix the module link brainpy-state.readthedocs.io/api/index.html -> canonical brainx.chaobrain.com/brainpy-state/apis/index.html (wrong host and wrong api/->apis/ path).
• docs/index.rst: fix the brainpy.state card link to the canonical URL.
• brainpy/state/README.md: add a relationship pointer and fix the State-based Documentation URL to canonical.

Validation

• sphinx -b dummy (notebook execution off): exit 0, no warnings on the edited files; all three canonical URLs return HTTP 200.

🤖 Generated with Claude Code

Summary by Sourcery

Clarify how brainpy.state relates to the classic BrainPy API and update state-related documentation and links to use the canonical brainpy-state URLs.

Enhancements:

• Clarify the relationship between the new State-based brainpy.state API and the classic DynamicalSystem-based BrainPy API in the state README.
• Update documentation links in the state README and Sphinx docs to point to the canonical brainpy-state site and relationship page.

Documentation:

• Add a relationship pointer between brainpy.state and the classic BrainPy API in the state README and API docs.
• Fix stale and incorrect brainpy-state documentation URLs in the README, docs index, and API reference to use the canonical host and paths.

[sourcery-ai]

Reviewer's guide (collapsed on small PRs)

Reviewer's Guide

Updates BrainPy state documentation to clarify the relationship between state-based and classic APIs and retargets all brainpy.state-related links to the new canonical docs host/paths.

File-Level Changes

Change	Details	Files
Clarify positioning of brainpy.state vs classic DynamicalSystem API and update state docs home URL to the canonical site.	Add a paragraph explaining that brainpy.state is the recommended entry point for new State-based differentiable models while the classic DynamicalSystem-based API remains supported. Link to the official relationship declaration page describing how state-based and classic paradigms relate. Change the State-based Documentation URL from the old readthedocs host to the canonical brainx.chaobrain.com/brainpy-state/ URL.	brainpy/state/README.md
Align brainpy.state references in Sphinx docs with the canonical brainpy-state documentation URLs and surface the relationship documentation.	In the API reference, add an admonition (relationship note) pointing to the brainpy.state relationship documentation so users understand how it fits with the rest of BrainPy. Fix the brainpy.state API link host from brainpy-state.readthedocs.io to brainx.chaobrain.com and correct the path from /api/ to /apis/. Update the brainpy.state card/link on the main docs index page to point to the new canonical documentation URL.	docs/api.rst docs/index.rst

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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