(ja_version) sync user_guide with main (full translation update + structure and warning fixes)

[hamataku]

Hello,

This PR updates the Japanese documentation under user_guide to align with the current main branch.

Changes include:

• Updating outdated pages to match recent content changes
• Translating newly added or modified sections
• Fixing Sphinx warnings

The translation/maintenance workflow was done with an AI agent, and I manually reviewed the results.

Thanks.

[duburcqa]

Note: This is a curated AI-generated review, verified and refined by a project maintainer.

Review: sync user_guide with main (full translation update + structure and warning fixes)

What's good

• Placeholder pages replaced with real content — concepts.md, collision_representation.md, sparse_computation.md were all "Coming soon..." stubs and are now either fleshed out or properly removed.
• taichi → quadrants migration in requirements.txt — correctly reflects the engine rename.
• .DS_Store cleanup — macOS metadata files properly removed.
• Heading levels fixed in hello_genesis.md — #### promoted to ##, correct for Sphinx rendering.
• Directory rename rigidentity/ → rigid_entity/ — better naming consistency.
• Many new translated user guide pages — sensors, SAP coupler, hybrid entities, manipulation, constraints, terrain, domain randomization, viewer plugins, recorders, etc.
• API reference restructured with proper hierarchy (engine, solvers, couplers, states, sensors, recording, visualization, differentiation, utilities, materials).

Issues

1. Delete deprecated IPC coupler doc rather than translating it.
source/user_guide/advanced_topics/ipc_coupler.md translates content that is already deprecated in the English main branch. This page should be removed entirely rather than shipped as new Japanese content. The same logic applies to any other pages that document APIs already deprecated in main — translating dead content creates maintenance burden and misleads users.

2. English API reference pages on a Japanese branch.
The ~60 new files under source/api_reference/ (differentiation, engine, couplers, sensors, recording, utilities, visualization) are entirely in English. If the ja_version branch is meant to be a Japanese documentation site, these should either be translated or explicitly marked as English-only. The inconsistency will be jarring for Japanese readers.

3. Commented-out HTML block in concepts.md.
There's a <!-- ... --> comment block that contains a near-duplicate of the paragraph immediately below it. This dead content should be removed.

4. differentiable_simulation.md content lost for Japanese readers.
The deleted page had substantive content about genesis.Tensor (creation patterns, scene attribute, zero_grad(), interaction with PyTorch). It was removed from the advanced topics toctree with no Japanese replacement — the new differentiable simulation content only exists in the English API reference. Japanese users lose this explanation.

[duburcqa]

Note: This is a curated AI-generated review, verified and refined by a project maintainer.

Review: sync user_guide with main (full translation update + structure and warning fixes)

What's good

• Placeholder pages replaced with real content — concepts.md, collision_representation.md, sparse_computation.md were all "Coming soon..." stubs and are now either fleshed out or properly removed.
• taichi → quadrants migration in requirements.txt — correctly reflects the engine rename.
• .DS_Store cleanup — macOS metadata files properly removed.
• Heading levels fixed in hello_genesis.md — #### promoted to ##, correct for Sphinx rendering.
• Directory rename rigidentity/ → rigid_entity/ — better naming consistency.
• Many new translated user guide pages — sensors, SAP coupler, hybrid entities, manipulation, constraints, terrain, domain randomization, viewer plugins, recorders, etc.
• API reference restructured with proper hierarchy (engine, solvers, couplers, states, sensors, recording, visualization, differentiation, utilities, materials).

Issues

1. Delete deprecated IPC coupler doc rather than translating it.
source/user_guide/advanced_topics/ipc_coupler.md translates content that is already deprecated in the English main branch. This page should be removed entirely rather than shipped as new Japanese content. The same logic applies to any other pages that document APIs already deprecated in main — translating dead content creates maintenance burden and misleads users.

2. English API reference pages on a Japanese branch.
The ~60 new files under source/api_reference/ (differentiation, engine, couplers, sensors, recording, utilities, visualization) are entirely in English. If the ja_version branch is meant to be a Japanese documentation site, these should either be translated or explicitly marked as English-only. The inconsistency will be jarring for Japanese readers.

3. Commented-out HTML block in concepts.md.
There's a <!-- ... --> comment block that contains a near-duplicate of the paragraph immediately below it. This dead content should be removed.

4. differentiable_simulation.md content lost for Japanese readers.
The deleted page had substantive content about genesis.Tensor (creation patterns, scene attribute, zero_grad(), interaction with PyTorch). It was removed from the advanced topics toctree with no Japanese replacement — the new differentiable simulation content only exists in the English API reference. Japanese users lose this explanation.

[hamataku]

Thank you for your review.

Issue 1
source/user_guide/advanced_topics/ipc_coupler.md still exists in the current main branch, and I could not find any indication that it has been deprecated. If I have missed something, please let me know and I will make the necessary changes.

Issue 2
This PR is focused on translating the user_guide. My understanding is that the api_reference is intended to remain in English, so I did not plan to translate those files.

Issue 3
The commented-out HTML block is already present in the English version on the main branch, and I translated it as-is for consistency. If you would prefer that it be removed, I am happy to update it.

Issue 4
advanced_topics/differentiable_simulation.md has already been removed in the upstream main branch and is no longer referenced in the main toctree. This PR does not remove the Japanese version independently; it follows the current state of main.