add fix for docutils in requirements

[SeverinDiederichs]

Same as https://github.com/ECP-WarpX/WarpX/pull/1884/files

(thanks to @ax3l)

This fixes an issue in our documentation were the logos appeared much larger than they used to do.

• Small enough (< few 100s of lines), otherwise it should probably be split into smaller PRs
• Tested (describe the tests in the PR description)
• Runs on GPU (basic: the code compiles and run well with the new module)
• Contains an automated test (checksum and/or comparison with theory)
• Documented: all elements (classes and their members, functions, namespaces, etc.) are documented
• Constified (All that can be const is const)
• Code is clean (no unwanted comments, )
• Style and code conventions are respected at the bottom of https://github.com/Hi-PACE/hipace
• Proper label and GitHub project, if applicable

[ax3l]

🎉