[draft] Token exchange docs

[rachel-carvalho]

What this PR does

All documentation updates related to token exchange.

Reviewer's guide to testing

Things to focus on

Checklist

Before submitting the PR, please consider if any of the following are needed:

• [ ] Update CHANGELOG.md if the changes would impact users
• [ ] Update README.md, if appropriate.
• [ ] Update any relevant pages in /docs, if necessary
• [ ] For security fixes, the Disclosure Policy must be followed.

[zzooeeyy]

Closing this since we added the commits to this PR instead

• https://github.com/Shopify/shopify_app/pull/1834