[polaris.shopify.com] Add the migration platform documentation to `/tools`

[chloerice]

tl;dr: Let's move the README for the migration tool to the style guide's new /tools section.

Open questions

• Should the docs be a single file? Routes per version/upgrade guide? Some other IA?

[SeanyB]

Should the docs be a single file? Routes per version/upgrade guide? Some other IA?

My initial take would be a page that provides all of the explanation and installation instructions, followed by separate pages for each version upgrade guide so that we don't end up with a single massive page.

We can ship with 70-80% confidence in an approach and then ask for feedback from some devs.

[sam-b-rose]

@chloerice, can we consider this closed? Looks like this is live https://polaris.shopify.com/tools/polaris-migrator

[sophschneider]

@samrose3 Finished this in #8087 but I didn't separate the migrations into separate pages. So if that work is still required I'd leave this open?

[sam-b-rose]

Good point. I think matching a similar way the Stylelint coverage rules are displayed would be good. Let's keep it open 👍 ty!

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