Add gatsby-plugin-layout docs to v2 migration post

[m-allanson]

Summary

There's a Gatsby v1 to v2 migration guide which covers breaking changes in v2.

It doesn't have any information about useful techniques for upgrading larger sites, particularly how to convert layouts. See related discussions below:

Refs: #5575 (review)
Refs: #3986 (comment)

Motivation

Upgrading to v2 should be straightforward for all Gatsby sites, large or small.

[shannonbux]

@amberleyromo in case this looks interesting!

[m-allanson]

Let's handle this with a new gatsby-plugin-layout plugin

[jlengstorf]

@m-allanson is there an issue for gatsby-plugin-layout? Or is this it?

[m-allanson]

The layout plugin will be added in #7204

[pieh]

There is one-liner in second bullet point in removal layouts section in migration guide - https://next.gatsbyjs.org/docs/migrating-from-v1-to-v2/#remove-or-refactor-layout-components which links to https://www.gatsbyjs.org/packages/gatsby-plugin-layout/ .

It would also be useful for this information to be available outside of migration guide, just not sure what's the best place for it.

[jlengstorf]

I'm closing this for now, because we don't really have a clear goal for how to write this out. It kind of seems like it could support a few narratives, e.g. "how to do page transitions" or "how to prevent component rerenders", but for now the scope of this issue is complete.