docs: add section on multiple layouts to gatsby-layout-plugin docs

[msachi]

Adding a section on how to use multiple layouts to gatsby-layout-plugin docs. Might be helpful to anyone trying to build a site with a more complex layout system (and who still needs to use or prefers the layout philosophy used in Gatsby v1).

[pieh]

I would move this to Troubleshooting section. This definitely need to be below v1 vs v2 explanation

[msachi]

No worries, have updated.

[pieh]

Thanks @msachi!

[DSchau]

This is great; thank you!

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