Gatsby: [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: https://github.com/gatsbyjs/gatsby/blob/38a865f7b5b25bbdedd51f2d0f6d8e45378e07e3/www/src/pages/docs/doc-links.yaml#L59-L72

[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!