Support physics and render configs in gym configuration

[yuecideng]

Description

This PR simplifies PhysicsCfg by removing four solver implementation options from the public configuration while retaining their established DexSim defaults.

It also adds top-level physics_config and render_cfg parsing to gym JSON/YAML files. Launcher-provided renderer values remain explicit overrides, while an omitted --renderer now preserves render_cfg.renderer.

The configuration guide and simulation manager documentation include the updated public surface and file-based configuration example.

Dependencies: None.

Issue: No issue linked.

Type of change

• Bug fix (non-breaking change which fixes an issue)
• Enhancement (non-breaking change which improves an existing functionality)
• New feature (non-breaking change which adds functionality)
• Breaking change (existing functionality will not work without user modification)
• Documentation update

Screenshots

Not applicable.

Testing

• pytest tests -q: 797 passed, 103 skipped
• pytest tests/sim/test_cfg.py tests/gym/utils/test_gym_utils.py -q: 30 passed
• Black passes for all files included in this PR
• git diff --check passes

The strict Sphinx -W build was attempted and reached the project-wide pre-existing documentation warnings/errors; no new warning was identified in the changed pages.

Checklist

• I have run Black to format the changed Python files.
• I have made corresponding changes to the documentation.
• I have added tests that prove the change works.
• Dependencies are unchanged.

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