Document how to run fourmolu without pre-commit

[deemp]

Details

Currently, there's only a script for running fourmolu via pre-commit.

https://github.com/objectionary/normalizer/blob/17dc8cbbba28b045f0c4640080df2251e6717861/site/docs/src/contributing.md?plain=1#L165

It'd be nice to have a script to run fourmolu without pre-commit while ignoring the same files as in the CI. Then, contributors without pre-commit will also be able to run fourmolu.

Subtasks

• [ ] Add a script for running fourmolu
• [ ] Make the script ignore the same files as in the CI
• [ ] Document in contributing.md