update examples

[chaoming0625]

This pull request includes several changes to the brainstate project, focusing on code simplification, functionality improvements, and documentation updates. The most important changes include removing redundant methods, updating import statements, and adding a new example script with licensing information.

Code simplification:

• brainstate/optim/_sgd_optimizer.py: Removed redundant extra_repr methods and simplified superclass initialization by using super() calls without specifying the class name. [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13]

Functionality improvements:

• brainstate/environ.py: Updated the register_default_behavior function to remove unused parameters and imports. [1] [2]
• brainstate/nn/_collective_ops.py: Improved the reset_all_states function by filtering nodes with the Module class.
• brainstate/nn/_interaction/_connections.py: Replaced jnp.dot with u.math.dot for matrix multiplication to maintain consistency.

Documentation updates:

• docs/index.rst: Updated documentation links from .rst to .html to ensure proper linking in the generated documentation. [1] [2] [3]

New example script:

• examples/200_surrogate_grad_lif.py: Added a new example script for reproducing results from the spytorch tutorial, including licensing information and detailed implementation for a spiking neural network.