docs: add webpack provider RFC

[ameerthehacker]

This contains the RFC to add webpack config provider, this came out of the tweet conversation with @KyleAMathews : twitter.com/kylemathews/status/1251249601539043328

[KyleAMathews]

Hey!

Thanks for writing this up. A few notes:

• Gatsby has a structured way to alter the webpack config as most of it is controlled directly by Gatsby — see the docs https://www.gatsbyjs.org/docs/node-apis/#onCreateWebpackConfig Often there's already Gatsby plugins for adding support for new webpack loaders e.g. https://www.gatsbyjs.org/packages/gatsby-plugin-less/?=less but generally speaking, your approach seems correct!
• we haven't updated this repo yet but we've moved RFCs into the main repo now https://github.com/gatsbyjs/gatsby/tree/master/rfcs

[ameerthehacker]

Thanks for the feedback @KyleAMathews, should I create a new PR in the main repo and shall I proceed with the development for the provider or do you think anything can be made better in this?