[README files] URLs should point to existing GitHub pages

[kaelig]

Issue

Steps to reproduce

1. go to https://github.com/Shopify/polaris-react/tree/master/src/components/Autocomplete
2. click "text field component"

Actual

💣 Links to https://github.com/Shopify/polaris-react/blob/master/components/forms/text-field

Expected

✅ Links to https://github.com/Shopify/polaris-react/tree/master/src/components/TextField

Proposal

For these URLs to work when people click on them in GitHub, this means URLs must be transformed in consuming projects: polaris-styleguide, and polaris-react's storybook.

Here's the proposed action plan:

• [polaris-react] (temporary) point to the styleguide itself, instead of having 404s in component descriptions: domainless styleguide format -> https://-prefixed styleguide format (See [Storybook] Direct example description links to https://polaris.shopify.com #933)
• [polaris-styleguide] Remove category in URL in styleguide (and install 302 redirects)
• Custom marked renderers for links:
  • [polaris-styleguide] Link to actual style guide pages
  • [polaris-react] Link to storybook stories
• [polaris-react] Update links in README files to be in "GitHub format

Once done, links will work on GitHub, polaris.shopify.com, and in Storybook 🎉

[kaelig]

Closing this as developing the tooling required to enforce those rules would take time we don't have right now.