Make field lists follow the style suggested by docutils' documentation

### Discussed in https://github.com/pradyunsg/furo/discussions/422

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<sup>Originally posted by **gunungpw** April 16, 2022</sup>


![image](https://user-images.githubusercontent.com/55311527/163676637-a4fe7ab5-5b85-4752-bbbe-be8d7b2347f2.png)

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