Add support for taxonomies

[JanLenoch]

Motivation

Kentico Kontent uses taxonomy groups to organize content. The GraphQL nodes of content items do not contain all taxonomy terms of all taxonomy groups. Just terms used in items.

Proposed solution

Introduce new Gatsby types with a KontentTaxonomy prefix. They should be interlinked with content items that use their taxonomy terms.

Reference

• https://docs.kontent.ai/reference/delivery-api#tag/Taxonomy-groups