Document GraphConfig JSON more clearly

[GCHQDeveloper314]

Currently the only direct mention of the GraphConfig JSON file is a brief section in the dev docs.

The graphConfig JSON file is important when deploying Gaffer graphs using the REST API (the usual means of deployment). As such, it should be documented in further detail to ensure that it's clear how this config file is used.

In the development guide, a full explanation of the GraphConfig Java object is given, but there's only a quick mention of the JSON version of this. A section should be added to the administration guide setting out the options for configuring a graphConfig.json file - this could link to the development page for further details.

[GCHQDeveloper314]

It might be better if some of the details related to this on the Graph component dev docs page were simply moved to the admin guide.