Fix ManiSkill docs build and extras install

[lola831]

Description

• Fix Sphinx docs when ManiSkill isn’t installed by:

  • Refactoring the existing LGSVL sphinx._buildingScenicDocs check into a shared
    buildingScenicDocumentation() helper and using it for both LGSVL and ManiSkill.
  • Mocking ManiSkill-related dependencies (mani_skill, torch, sapien,
    gymnasium) in autodoc_mock_imports.
  • Registering the ManiSkill ScenicEnv only when not building docs.

• Make scenic[maniskill] installable without pip’s resolution-too-deep error by
  tightening the extra’s gymnasium requirement to gymnasium >= 1.0.0.

Issue Link

N/A

Checklist

• I have tested the changes locally via pytest and/or other means
• I have added or updated relevant documentation
• I have autoformatted the code with black and isort
• I have added test cases (if applicable)

Additional Notes