docs: link the BrainPy to BrainX guide and migration notes

[chaoming0625]

The BrainX docs now carry two pages written for brainpy users:

• BrainPy to BrainX — how
  brainpy and the BrainX packages relate, which package owns which modeling scale, and
  whether a given project should move
• BrainPy migration notes —
  the mechanics: brainpy vs brainpy.state, runtime and module mapping tables, a worked
  before/after E/I network, and what does not carry over

Nothing in this repository pointed at either, so brainpy readers never reached them.
This adds links in the four places a reader would plausibly look.

Changes

• README.md — the concept page in the header link block; both pages in the ecosystem list
• docs/index.rst — a Learn more card, next to the existing brainpy.state APIs card
• docs/api.rst — extends the brainpy.state admonition added in docs: link brainpy.state relationship and fix stale doc URLs #873; someone hunting
  the API reference for a moved symbol is exactly who needs the mapping tables
• brainpy/state/README.md — both pages under Documentation

Links only — no existing URLs changed.

Verification

• sphinx-build over docs/ exits 0 with no warning from index.rst or api.rst
  (the 534 pre-existing warnings are unrelated)
• all three linked URLs return 200

🤖 Generated with Claude Code

Summary by Sourcery

Add documentation cross-links to guide BrainPy users towards the new BrainX overview and migration notes.

Documentation:

• Link the BrainPy-to-BrainX guide and migration notes from the main README ecosystem section and header links.
• Expose the BrainPy-to-BrainX guide and migration notes from the brainpy.state README documentation links.
• Add pointers in the Sphinx docs index and API pages to the BrainPy-to-BrainX overview and migration resources for users searching the state-based API.

[sourcery-ai]

Reviewer's guide (collapsed on small PRs)

Reviewer's Guide

Adds prominent documentation links guiding BrainPy users to the new BrainX ecosystem, including conceptual explanation and migration notes, by updating top-level README, state README, and Sphinx docs entry points.

File-Level Changes

Change	Details	Files
Expose BrainPy→BrainX conceptual and migration docs from the top-level project README.	Add a new header link to the BrainPy-to-BrainX conceptual page alongside existing documentation and ecosystem links. Extend the ecosystem/resources list with a BrainPy-to-BrainX entry that also inlines a link to the migration notes page and briefly describes their purpose.	README.md
Expose BrainPy→BrainX docs from the brainpy.state package README for state-based users.	Add links to the BrainPy-to-BrainX conceptual page and the migration notes page under the Documentation section so state-based users can discover cross-ecosystem guidance.	brainpy/state/README.md
Surface migration guidance from Sphinx docs entry points for users browsing HTML docs.	Add a new “Learn more” card on the documentation index pointing to the BrainPy-to-BrainX conceptual page, colocated with the existing brainpy.state APIs card. Extend the brainpy.state-related admonition in the API reference to mention and link to the migration notes and/or BrainPy-to-BrainX docs for users searching for moved symbols.	docs/index.rst docs/api.rst

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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