docs: Add Renovate configuration.
docs: Add Renovate configuration.

While some repositories which host Hexo docs alongside other code may not have
Renovate setup, it's anticipated that they will in the future, at the very
least for the docs portions of the respective repositories.

By having this `renovate` section in the `package.json` for the Hexo docs, we
can ensure that they will receive the proper treatment and minimize the amount
of manual work necessary on those repositories.

Additionally, this ensures that there is a single point of upstream Apollo-docs
Renovate configuration through the use of the
[`renovate-config-apollo-docs`](https://npm.im/renovate-config-apollo-docs)
package, which inherits further from
[`renovate-config-mdg-docs`](https://npm.im/renovate-config-mdg-docs).

Repositories which already have Renovate setup should benefit automatically
from this new docs-specific configuration.

apollographql/apollo-link,apollographql/apollo-link