Bump underscore from 1.12.0 to 1.13.1 in /tools/doc (#6985)

Bumps [underscore](https://github.com/jashkenas/underscore) from 1.12.0 to 1.13.1.
- [Release notes](https://github.com/jashkenas/underscore/releases)
- [Commits](jashkenas/underscore@1.12.0...1.13.1)

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