Add a new photo to the mobile food vendor page

**What is the URL?**
[Mobile food vendor guide](https://alpha.austin.gov/en/permits-tickets/business-permits-and-licenses/food-business-permits/mobile-food-vendor-permit-guide/)

**Have you documented this via the author notes field in Joplin?**
- [ ] Yes!
- [x] No, but I'll do it now

**Describe the content debt.**
Let's add a photo that illustrates happy mobile food vendors with a permitted truck!

**What is the long-term, ideal solution for this content debt?**
Get a hi-res great photo with any required releases

**Is this related to a feature request issue? (Add the issue number below and add the issue as a dependency.)**

**What approvals will be needed to publish this content change?**
APH will need to sign off on any photo we choose to put up. (PIO)