docs: example of modifying babel loader (#7739)
docs: example of modifying babel loader (#7739)

There used to be an example of how to do this in v1 and people are having
trouble migrating without it.  It would be nice if there was a proper equivalent
of v1's `config.removeLoader("js")` so that it's not necessary to filter by the
`test` rule.
mingaldrichgan/gatsby,0x80/gatsby,ChristopherBiscardi/gatsby,gatsbyjs/gatsby,ChristopherBiscardi/gatsby,gatsbyjs/gatsby,0x80/gatsby,mingaldrichgan/gatsby,0x80/gatsby,mingaldrichgan/gatsby,ChristopherBiscardi/gatsby,gatsbyjs/gatsby,gatsbyjs/gatsby,gatsbyjs/gatsby,ChristopherBiscardi/gatsby,gatsbyjs/gatsby