Add BrainPy citation and update documentation

[chaoming0625]

This pull request includes updates to the README.md and documentation files to improve clarity, provide updated references, and enhance usability. The most important changes include updating references to BrainPy's ecosystem and documentation, adding citation details for the BrainPy framework, and reorganizing the API documentation structure.

Updates to README.md:

• Updated the description of BrainPy's foundation to specify that it has been rewritten based on brainstate since August 2025.
• Replaced the outdated link to brainpy-datasets with a more general reference to the "brain modeling ecosystem."
• Added a detailed BibTeX citation for BrainPy, including publication details in the journal eLife, for users to reference the framework in academic work.

Documentation improvements:

• Removed changelog links (brainpy-changelog.md and brainpylib-changelog.md) from the docs/api.rst file, likely to streamline the API documentation.
• Added an "Ecosystem" installation tab in docs/index.rst with commands for installing BrainX on different platforms (CPU, CUDA, TPU).
• Updated the "Learn more" section in docs/index.rst to include changelog links and fixed an outdated example link. [1] [2]

Summary by Sourcery

Enhance README and documentation by updating BrainPy references, adding citation details, and improving the ecosystem installation and API docs structure

New Features:

• Add detailed BibTeX citation for the BrainPy framework in README.md
• Introduce an "Ecosystem" installation tab with CPU, CUDA, and TPU commands in docs/index.rst

Enhancements:

• Update README to specify rewriting based on brainstate since August 2025
• Replace the brainpy-datasets link with a generic brain modeling ecosystem reference

Documentation:

• Remove obsolete changelog links from docs/api.rst
• Update "Learn more" section in docs/index.rst with corrected example and changelog links

[sourcery-ai]

Reviewer's Guide

Enhance project documentation by refining the README foundation description, replacing outdated links with a general ecosystem reference, embedding a detailed BibTeX citation, removing stale changelog links from the API docs, and extending the index rst with an Ecosystem installation tab and updated Learn More section.

File-Level Changes

Change	Details	Files
Enhanced README foundation description and citation details	Clarified rewrite origin with “since August 2025” Replaced brainpy-datasets link with a generic ecosystem reference Inserted detailed BibTeX citation block for academic referencing	README.md
Streamlined API documentation by removing outdated links	Deleted brainpy-changelog.md link Deleted brainpylib-changelog.md link	docs/api.rst
Reorganized landing page with an Ecosystem install tab and refreshed Learn More section	Added Ecosystem installation tab with CPU, CUDA, and TPU commands Updated Learn More section with changelog links Fixed outdated example link	docs/index.rst

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[sourcery-ai]

Hey @chaoming0625 - I've reviewed your changes and they look great!

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[chaoming0625]

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