[Newton] Removes most docs and fix doc building

[kellyguo11]

Description

Keeping only the API docs on the dev/newton branch for now since most other docs are currently irrelevant.
Fixes build issues with docs.

Type of change

• Documentation update

Checklist

• I have read and understood the contribution guidelines
• I have run the pre-commit checks with ./isaaclab.sh --format
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• I have updated the changelog and the corresponding version in the extension's config/extension.toml file
• I have added my name to the CONTRIBUTORS.md or my name already exists there

[greptile-apps]

Skipped: This PR changes more files than the configured file change limit: (121 files found, 100 file limit)