Add discount functions settings api docs

[devisscher]

Background

This PR adds a section for Discount Function Settings API, similar to the Validation Settings API.

Related PR: https://github.com/Shopify/web/pull/151973

Solution

Adding the interface and data provided to the extension point.

🎩

Spin Link

For info on how to setup docs: https://app-ui.docs.shopify.io/guides/documentation/ui-extensions

1. Go to Discount Function Settings API and review content.

Checklist

• I have 🎩'd these changes
• I have updated relevant documentation

[iphipps]

looks good. Do you also need to generate the docs and pr on shopify-dev?

[MitchLillie]

Hey Tom. You will need to add a changeset (yarn changeset, pick minor, commit the resulting file).

As Ian mentioned, after this is merged you will need to regenerate the docs and create a PR on shopify-dev

[olavoasantos]

Does this need to be a const? I feel it might be ideal to keep this as a type to avoid increasing the runtime size of the library.

[devisscher]

Updated and moved the SupportedDefinitionTypes to shared since they are used in Validation Settings as well.

[devisscher]

Hey Tom. You will need to add a changeset (yarn changeset, pick minor, commit the resulting file).

As Ian mentioned, after this is merged you will need to regenerate the docs and create a PR on shopify-dev

@MitchLillie I added the changeset.