Support relationships between content items and their modular items

[JanLenoch]

Motivation

Kentico Cloud content items can be composed of other content items, so called modular content items. They can be put into modular content elements or into the rich text elements.

Design guidelines

Bring support for modular content linking by adding new navigation properties to content item nodes. Make use of the ___NODE convention to tell Gatsby that a field contains an ID of a linked node.

Reference

• How to manually set graphql type / link to existing nodes gatsbyjs/gatsby#1583
• https://github.com/gatsbyjs/gatsby/blob/master/packages/gatsby-source-filesystem/README.md
• https://github.com/Kentico/cloud-gatsby/blob/master/plugins/gatsby-source-kentico-cloud/normalize.js

[JanLenoch]

Implemented via 825080f and 6d74079 .