docs: link the BrainPy to BrainX guide and migration notes

The BrainX docs now carry two pages for brainpy users: a concept page on
how the packages relate, and migration notes with the module and runtime
mappings. Nothing in this repository pointed at them, so brainpy readers
never reached them.

- README: add the concept page to the header links and both pages to the
  ecosystem list
- docs/index.rst: add a Learn more card next to the brainpy.state APIs card
- docs/api.rst: extend the brainpy.state admonition, where a reader hunting
  for a moved symbol will look
- brainpy/state/README.md: add both pages under Documentation

Author: chaoming0625

README.md

@@ -21,6 +21,7 @@ BrainPy is a flexible, efficient, and extensible framework for computational neu
 - **Documentation**: https://brainpy.readthedocs.io/
 - **Documentation (state-based)**: https://brainx.chaobrain.com/brainpy-state
 - **Ecosystem**: https://brainx.chaobrain.com
+- **BrainPy and BrainX**: https://brainx.chaobrain.com/summ/brainpy-to-brainx.html
 
 
 ## Installation
@@ -64,6 +65,7 @@ We provide a Binder environment for BrainPy. You can use the following button to
 - **[brain modeling ecosystem](https://brainmodeling.readthedocs.io/)**: A collection of tools and libraries for brain modeling and simulation.
 - **[brainpy-examples](https://github.com/brainpy/examples)**: Comprehensive examples of BrainPy computation. 
 - **[brainpy-state](https://github.com/chaobrain/brainpy.state)**: Moderize BrainPy simulation with `brainstate` syntax.
+- **[BrainPy to BrainX](https://brainx.chaobrain.com/summ/brainpy-to-brainx.html)**: How `brainpy` relates to the BrainX packages, and [migration notes](https://brainx.chaobrain.com/summ/brainpy-migration-notes.html) for moving a project across.
 - [《神经计算建模实战》 (Neural Modeling in Action)](https://github.com/c-xy17/NeuralModeling)
 - [第一届神经计算建模与编程培训班 (First Training Course on Neural Modeling and Programming)](https://github.com/brainpy/1st-neural-modeling-and-programming-course)
 - [第二届神经计算建模与编程培训班 (Second Training Course on Neural Modeling and Programming)](https://github.com/brainpy/2nd-neural-modeling-and-programming-course)

brainpy/state/README.md

@@ -21,6 +21,8 @@ For comprehensive documentation on state-based programming in BrainPy, please vi
 
 - **State-based Documentation**: https://brainx.chaobrain.com/brainpy-state/
 - **Main BrainPy Documentation**: https://brainpy.readthedocs.io/
+- **BrainPy and BrainX**: https://brainx.chaobrain.com/summ/brainpy-to-brainx.html
+- **Migration notes**: https://brainx.chaobrain.com/summ/brainpy-migration-notes.html
 
 ## Source Repository
 

docs/api.rst

@@ -32,6 +32,12 @@ API Documentation
    and fully supported. See the `brainpy.state relationship page
    <https://brainx.chaobrain.com/brainpy-state/project/relationship.html>`_.
 
+   Looking for where a ``brainpy`` symbol lives in the wider ecosystem? The
+   `BrainPy migration notes <https://brainx.chaobrain.com/summ/brainpy-migration-notes.html>`_
+   map the classic modules onto their BrainX counterparts, and
+   `BrainPy to BrainX <https://brainx.chaobrain.com/summ/brainpy-to-brainx.html>`_
+   explains how the packages relate.
+
 The following APIs will no longer be maintained in the future, but you can still use them normally.
 
 .. toctree::

docs/index.rst

@@ -120,6 +120,13 @@ Learn more
          :class-card: sd-text-black sd-bg-light
          :link: https://brainx.chaobrain.com/brainpy-state/
 
+   .. grid-item::
+      :columns: 6 6 6 4
+
+      .. card:: :material-regular:`swap_horiz;2em` BrainPy and BrainX
+         :class-card: sd-text-black sd-bg-light
+         :link: https://brainx.chaobrain.com/summ/brainpy-to-brainx.html
+
 
 ----