[update] Refine installation instructions and improve docstring formatting

[chaoming0625]

This pull request includes updates to the documentation, codebase, and dependency management for the BrainPy project. The changes focus on improving clarity, updating installation instructions, and ensuring consistency with the project's recent rewrite based on the BrainState framework.

Documentation Updates:

• Updated the license badge in README.md to use a GNU GPLv3 badge and removed references to OpenI. Added a note about BrainPy's rewrite based on BrainState and updated the Python version requirement to 3.10. [1] [2]
• Simplified installation instructions in docs/index.rst by removing dependency links for GPU and TPU installations and added a note about the BrainState rewrite. [1] [2]
• Removed the installation section from the quickstart table of contents in docs/index.rst.

Codebase Improvements:

• Updated docstrings across multiple files (lowdim_analyzer.py, lowdim_bifurcation.py, random_conn.py) to use raw string literals (r""") for better rendering of mathematical expressions in documentation. [1] [2] [3] [4] [5] [6] [7]

Dependency Management:

• Removed unused dependency_links from setup.py and corrected the TPU extras requirement to include brainstate[tpu] instead of brainstate[cuda12].