Documentation: Integrate Polaris documentation with Hub

[jstlaurent]

Context

Since the Hub and the library are tightly integrated, we want our users to easily access both documentation. This will need some tweaks to the way we usually setup documentation for our libraries.

Description

1. Host the docs on the Polaris Hub domain (ref), using a subdomain.

2. Extend the theme to add a custom header that lets you easily navigate back to the hub or other doc sites we might generate.

Acceptance Criteria

• Users can access the library documentation through a subdomain of the Polaris apex domain
• Users have a link to navigate back to the Hub documentation

Links

• Slack conversation