Introduction wording on index page could be improved

The current wording on the main index page could be improved:

> The Brand Store is a marketplace engine for software.
>
> It provides a platform for publishers to release and distribute software
> artefacts and for consumers to discover, deploy and update them, including in
> environments with limited or no network and compatibility with any
> Linux-based OS or device. The Brand Store also offers the option to publish private snaps.

The Brand Store doesn't really provide a platform for publishers, nor does it allow consumers to discover, deploy and update. That sounds more like a description of the public Snap Store. The Brand Store is primarily meant to provide a private snap store for customers and ensure that snaps hosted in it can **only** be installed on the brand's own devices. It also provides a means for brands to empower snaps by granting them rights to specific snapd interfaces.

I also question our inclusion of the phrase "any Linux-based OS or device". AFAIK we only support Brand Stores for Ubuntu Core devices (soon to be Core based Desktop devices too), and while can support Brand Stores for Ubuntu Classic, I'm not aware of an actual customer engagement where we've done so (this doesn't mean it hasn't happened).

My suggestion would be to go back to the original introduction from the [Brand Store Onboarding for UC22 [TEMPLATE]](https://docs.google.com/document/d/11z7iKogO7FDouJBfYgh9hROK41xDeaPy0ruS2_flyL0/edit) document which simply says:

> Welcome to your Brand Store.
>
> When using your Brand Store, your main resource will be the documentation found at https://ubuntu.com/core/services/guide/iot-app-store-intro. In this guide, we will provide links to some key pages in the documentation, as well as information specific to your Brand Store.
>
> This is the start of your journey with snaps and Snap Stores. We look forward to working with you to ensure your commercial success.
>

Start by locating the main index page for the Brand Store documentation and compare its introduction with the wording proposed in this issue. Update the introduction to describe the private Brand Store purpose, supported scope, and customer guidance accurately, then review the rendered page for clear wording.

Written by the indexing model from the issue text.