Refactor V2 tests to tests_version2 and add V2/V3 documentation READMEs

[oujago]

Summary by Sourcery

Harmonize BrainPy version2 test suite by renaming the tests directory to tests_version2 and update all related references in code, CI, documentation, and examples, while adding dedicated README files for version2 and version3 documentation and examples

New Features:

• Add README for version2 examples
• Introduce documentation README for BrainPy version2 and version3
• Add README for version3 examples

Enhancements:

• Rename test suite directory and references from tests to tests_version2 across codebase, comments, notebooks, and example scripts

CI:

• Update CI workflows to run pytest against tests_version2

Documentation:

• Update contributing guidelines, PR template, labeler config, and tutorials to reference tests_version2

[sourcery-ai]

Reviewer's Guide

This PR standardizes the Version 2 test suite by renaming all test references from "tests" to "tests_version2" across CI workflows, documentation, and code comments, and augments the repository with dedicated example and documentation READMEs for both Version 2 and Version 3.

File-Level Changes

Change	Details	Files
Rename and reroute test suite invocations to tests_version2	Swap out pytest commands from tests/ to tests_version2/ Update CI-models and CI workflows to invoke pytest tests_version2/ Adjust CONTRIBUTING and tutorial docs to run tests_version2/ Revise pull request template and labeler globs to reference tests_version2/	.github/workflows/CI-models.yml .github/workflows/CI.yml CONTRIBUTING.md docs_version2/tutorial_advanced/contributing.md .github/PULL_REQUEST_TEMPLATE.md .github/labeler.yml
Add version2 suffix to inline comments in test and parsing code	Append _version2 to comments describing test harnesses and flaky tests Update comment references for issues and test inspirations to include _version2 suffix	brainpy/version2/math/tests/test_numpy_ops.py brainpy/version2/math/tests/test_einops.py brainpy/version2/math/tests/test_environment.py brainpy/version2/math/tests/test_numpy_einsum.py brainpy/version2/running/tests/test_pathos_multiprocessing.py brainpy/version2/math/einops_parsing.py
Introduce examples_version2 README	Create a top-level README documenting Version 2 examples and usage notes	examples_version2/README.md
Introduce Version 3 documentation and examples	Add docs_version3 README outlining tutorials and API content Add examples_version3 README cataloguing Version 3 examples	docs_version3/README.md examples_version3/README.md

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[sourcery-ai]

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

Prompt for AI Agents

Please address the comments from this code review:

## Individual Comments

### Comment 1
<location> `examples_version2/README.md:65-68` </location>
<code_context>
+Spiking neural network training examples:
+
+- **spikebased_bp_for_cifar10.py** - Spike-based backpropagation for CIFAR-10
+- **readme.md** - Additional SNN training documentation
+
+## Requirements
</code_context>

<issue_to_address>
**suggestion:** 'readme.md' is listed as an example script, but it's a documentation file.

Please remove 'readme.md' from the example scripts list to avoid confusion.

```suggestion
Spiking neural network training examples:

- **spikebased_bp_for_cifar10.py** - Spike-based backpropagation for CIFAR-10
```
</issue_to_address>

### Comment 2
<location> `examples_version3/README.md:20` </location>
<code_context>
+- **104_CUBA_2005.py** - Current-based E/I network (CUBA model)
+- **106_COBA_HH_2007.py** - COBA network with Hodgkin-Huxley neurons
+- **107_gamma_oscillation_1996.py** - Gamma oscillation generation
+- **108_synfire_chains_199.py** - Synfire chain propagation
+- **109_fast_global_oscillation.py** - Fast global oscillation dynamics
+
</code_context>

<issue_to_address>
**issue (typo):** Filename '108_synfire_chains_199.py' may contain a typo in the year.

Please verify and correct the year in the filename to ensure consistency with the other files.

```suggestion
- **108_synfire_chains_1991.py** - Synfire chain propagation
```
</issue_to_address>

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[sourcery-ai]

suggestion: 'readme.md' is listed as an example script, but it's a documentation file.

Please remove 'readme.md' from the example scripts list to avoid confusion.

Suggested change

	Spiking neural network training examples:
	- **spikebased_bp_for_cifar10.py** - Spike-based backpropagation for CIFAR-10
	- **readme.md** - Additional SNN training documentation
	Spiking neural network training examples:
	- **spikebased_bp_for_cifar10.py** - Spike-based backpropagation for CIFAR-10

[sourcery-ai]

issue (typo): Filename '108_synfire_chains_199.py' may contain a typo in the year.

Please verify and correct the year in the filename to ensure consistency with the other files.

Suggested change

	- **108_synfire_chains_199.py** - Synfire chain propagation
	- **108_synfire_chains_1991.py** - Synfire chain propagation

[oujago]

@sourcery-ai title