Update Gatsby v1-to-v2 guide to include add'l guidance on removal of layouts

[bw]

This is an update to the v1-to-v2 guide in the docs, adding more clarity on the design shift from layouts-wrap-pages to pages-wrap-components. It includes a link to the ongoing discussion in #6127.

I also flipped, within that section of the guide, the order of #4 and #5. It seems to me that it's more likely that someone is using props in layouts than using queries. This order roughly prioritizes the tasks for upgrading components by order of likelihood.

[KyleAMathews]

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[KyleAMathews]

Thanks! Nicely written and I'm sure people will appreciate the clarifications here while upgrading.

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