Improve the wording of "Browse UTM Gallery" in the UI (it's not self-evident what "UTM Gallery" actually means)

**Describe the issue**
The UI assumes that first-time users somehow automatically know that "UTM Gallery" actually means "a place where users can download prebuilt VM images".

This is what every brand new user sees after launching UTM for the very first time:
(Browse "**_UTM Gallery_**"?? Why? What is that?)

Suggestion: replace "Browse UTM Gallery" with something more descriptive, such as:
- "Download a Prebuilt VM" (or "Virtual Machine")
- "Download a Ready-Made VM"
- "Download a VM Image"
- "Browse and Download VMs"
- "Explore VM Images"
- "Discover Ready-Made VMs"
- "Browse VM Catalog"
- "Browse VM Image Library"
- "Browse VM Image Gallery"
- "Download a VM image from UTM Gallery"

...or something like that.

PS. I think the **download** icon is an excellent choice here (it correctly implies that something will be downloaded)