Add discount extension, move some files so helpers work

[NathanJolly]

Background

Resolves https://github.com/Shopify/pos-next-react-native/issues/36272

Solution

This adds the example discount extension static page. It also moves the pages into staticPages/pages so that the generateCodeBlock helper works for both pages in reference as well as staticPages.

I also fixed a typo in the cart api and found I had missed a section in the getting-started page. I also updated a couple of links.

🎩

Old: https://shopify.dev/docs/apps/pos/ui-extensions/discount-extension New: https://shopify-dev.ui-extensions-70k7.nathan-oliveira.us.spin.dev/docs/api/pos-ui-extensions/unstable/example-discount-extension

Checklist

• [x] I have :tophat:'d these changes
• [x] I have updated relevant documentation

[github-actions[bot]]

We detected some changes in packages/*/package.json or packages/*/src, and there are no updates in the .changeset directory. If the changes are user-facing and should cause a version bump, run yarn changeset to track your changes and include them in the next release CHANGELOG. If you are making simple updates to repo configuration, examples, or documentation, you do not need to add a changeset.