Dark mode icon (#4478)

## What are you changing in this pull request and why?

Logo disappeared on the /guides landing page when in dark mode 
- Copied logo to /white dir
- Updated Public Preview callout 

## Checklist
- [x] Review the [Content style
guide](https://github.com/dbt-labs/docs.getdbt.com/blob/current/contributing/content-style-guide.md)
and [About
versioning](https://github.com/dbt-labs/docs.getdbt.com/blob/current/contributing/single-sourcing-content.md#adding-a-new-version)
so my content adheres to these guidelines.

website/docs/guides/microsoft-fabric-qs.md

@@ -20,7 +20,7 @@ In this quickstart guide, you'll learn how to use dbt Cloud with Microsoft Fabri
 
 :::tip Public preview
 
-A public preview of dbt Cloud support for Microsoft Fabric in now available! 
+A public preview of Microsoft Fabric in dbt Cloud is now available! 
 
 :::