[docs] Continue to flesh out gatsby-node api functions descriptions

[lannonbr]

Summary

We have currently documented each of the various Node APIs here: https://www.gatsbyjs.org/docs/node-apis

That said, a large portion of them are just the name and a small description of it. What would be useful would be to add further context about each one that can include things like

• Parameters specific to this API
• an example of using the API

Basic example

the onCreateWebpackConfig API is a good example to describe what the API is, what params are passed in, and an example of using it.

https://www.gatsbyjs.org/docs/node-apis/#onCreateWebpackConfig

[sampittko]

I would like to document onCreatePage API, should I create another issue referencing this one but more specific?