Create migration guidance for `polaris-tokens` package

[alex-page]

transitive dependencies (global-nav and online-store-ui)

grep '@shopify/polaris-tokens' node_modules/**/package.json

[BPScott]

Running grep '@shopify/polaris-tokens' node_modules/**/package.json in web's codebase shows packages that use tokens within web or its dependencies.

grokt may be handy for curating list of repos across the org: A search for references to @shopify/polaris-tokens in package.json files. There's probably something you can do with ServiceDB announcements to raise an issue in multiple repositories at the same time