Updates add-custom-webpack-config.md to fix broken links (#1420)
Updates add-custom-webpack-config.md to fix broken links (#1420)

* Updates add-custom-webpack-config.md to fix broken links

* Corrects links in add-custom-webpack-config.md

* Updates webpack.config.js to relative path for add-custom-webpack-config.md

* fix links

Khaledgarbaya/gatsby,mingaldrichgan/gatsby,gatsbyjs/gatsby,ChristopherBiscardi/gatsby,gatsbyjs/gatsby,ChristopherBiscardi/gatsby,mickeyreiss/gatsby,ChristopherBiscardi/gatsby,mickeyreiss/gatsby,fk/gatsby,fabrictech/gatsby,fk/gatsby,0x80/gatsby,mickeyreiss/gatsby,0x80/gatsby,Khaledgarbaya/gatsby,danielfarrell/gatsby,ChristopherBiscardi/gatsby,fabrictech/gatsby,chiedo/gatsby,gatsbyjs/gatsby,gatsbyjs/gatsby,0x80/gatsby,danielfarrell/gatsby,mingaldrichgan/gatsby,mingaldrichgan/gatsby,chiedo/gatsby,gatsbyjs/gatsby,danielfarrell/gatsby,Khaledgarbaya/gatsby,fk/gatsby,gatsbyjs/gatsby,chiedo/gatsby,fabrictech/gatsby