New Example on creating a custom `GradientTransformation`

### Motivation

Recently for a personal project (#641) I wanted to created a custom `GradientTransformation` object and found out that there wasn't a tutorial or walkthrough on how to create one. While the ["Gradient Transformations"](https://github.com/google-deepmind/optax#gradient-transformations-transformpy) section in the `README.md` section was somewhat helpful, I still had to look around and search for example implementations in other repositories and within the `contrib` sub-package.

### Proposal

Create a new example or a page in the documentation containing instructions on how to create a custom `GradientTransformation` object.

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Happy to contribute and work on this.