[Docs] Clarify Admin REST Docs

[lizkenyon]

WHAT is this pull request doing?

• Improve Admin REST docs based on developer feedback

• Point developers to the Admin REST references docs, to see specific example there using the REST resources

• Move the app.routes file example with the relevant code to be shown in the first tab and the supporting code in shopify.server to the second tab

• Add a POST request example. Because we are not pulling in the types from Shopify-api-js currently, it is not easy to determine the params.

• Also forgot to run yarn build-docs yesterday to webhook updates are included here. 🙈

Type of change

• Patch: Bug (non-breaking change which fixes an issue)
• Minor: New feature (non-breaking change which adds functionality)
• Major: Breaking change (fix or feature that would cause existing functionality to not work as expected)

Checklist

• I have used yarn changeset to create a draft changelog entry (do NOT update the CHANGELOG.md files manually)
• I have added/updated tests for this change
• I have documented new APIs/updated the documentation for modified APIs (for public APIs)