[gatsby-config.js] Documentation needed

[EvansJahja]

Following #2032, We need to have docs for gatsby-config.js.

It seems Joi is used for validating the schema and the spec can be derived from here.

[calcsam]

Yes, this should be a new page living under "Reference" in the docs. Want to add this documentation @Charon77?

[EvansJahja]

Sure thing @calcsam
By the way, how to make the pages appear in the left sidebar? Is there a guide on how to add to the gatsby reference?

[fk]

Hey @Charon77! Add your new Markdown document here: https://github.com/gatsbyjs/gatsby/tree/master/docs/docs, then link it there:

gatsby/www/src/pages/docs/doc-links.yaml

Lines 59 to 72 in 38a865f

	- title: Reference
	items:
	- title: Node Interface
	link: /docs/node-interface/
	- title: API Specification
	link: /docs/api-specification/
	- title: Bound Action Creators
	link: /docs/bound-action-creators/
	- title: Gatsby Browser APIs
	link: /docs/browser-apis/
	- title: Gatsby Node APIs
	link: /docs/node-apis/
	- title: Gatsby SSR APIs
	link: /docs/ssr-apis/

[amberleyromo]

Hey @Charon77, are you still adding these docs?

[EvansJahja]

@amberleyromo honestly I hadn't got the time to even start. Feel free to take over 😄
I'll add stuffs if I can.

[KyleAMathews]

@amberleyromo's PR adding docs are in! Thanks!