Change text content of the root page, to avoid user confusion

[stevejalim]

Description

In a new deployment of Wagtail within Bedrock, the top-level page (at /) is never served because we already have a Django view for the homepage that matches that route well before Wagtail gets to it. We can't remove it, because then the next level down would become the root page and therefore not get served either.

It's likely the homepage will become CMS-managed, but we need to decide whether that'll be the whole page or Wagtail Snippets mixed in with hard-coded content. For now, the base/root/homepage at / inside the CMS is not the real www.mozilla.org root page.

To help avoid confusion, we could re-title the top-level page from "Welcome to your new Wagtail site!" to something clearer. e.g:

Doing this via a migration means it'll be available everywhere during the development and rollout process.

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[stevejalim]

Easier to just do live and then export a DB for local dev