`Creating Ubuntu Core images` is now described as a tutorial

[Brand Store Onboarding for UC22 [TEMPLATE]](https://docs.google.com/document/d/11z7iKogO7FDouJBfYgh9hROK41xDeaPy0ruS2_flyL0/edit) has a section titled `Creating Ubuntu Core images` which is the second to last section in the document (with Snap confinement being last).

In this new project it's now the first section (with the link labeled as `Tutorial`) provided on the [index](https://github.com/canonical/brand-store-documentation/blob/main/index.rst) page, preceded by text that reads:

> If you are not yet familiar with Brand Store workflows, start with our tutorial:

This doesn't seem correct to me. First, the reason the section is provided isn't to familiarize customers with brand store workflows, it's purpose is to ensure that customers have reference for building Ubuntu Core images that properly onboard and optionally include snaps from the customer's Brand Store. Every single customer needs to build images, and our public documentation doesn't really provide a guided set of instructions for doing so.

The reason is was originally placed towards the end of the doc is that in order to create gadgets, models, and images, your brand store needs to already be configured (i.e. someone needs an SSO account with viewer privileges in the Brand Store in order to download Brand Store hosted snaps to include in an image), and thus it made sense to put this _after_ the configuration section.

Start with index.rst and inspect how the “Creating Ubuntu Core images” section is linked and introduced. Compare that placement and wording with the onboarding sequence described in the issue and the referenced Brand Store onboarding document. Done means the index accurately explains the section’s purpose and places it consistently with its prerequisites.

Written by the indexing model from the issue text.