Serve govuk-component documentation as JSON

Previously this documentation was stored in the govuk_component_guide app and was
frequently out of sync with static, or failed if pointing at a different static
endpoint.

It's used to generate the living documentation of https://github.com/alphagov/govuk_component_guide

This moves the documentation into static and exposes it as an API-like endpoint,
so that the documentation data could be auto generated in the future, rather than
a static file.

The major change here is moving the docs data into static so it stays in sync, and is exposed by a reliable endpoint. The way it's structured internally can easily be changed in the future.