For vendors: Add to nav

[machikoyasuda]

closes #500

What this PR does

• Adds one link to the nav
• Any CSS changes? No.

[machikoyasuda]

@thekaveman @angela-tran This is ready for review. Since we're going to be going live when all the PRs are in, this PR can be safely merged into the 499-for-vendors-newsletter branch.

[angela-tran]

@machikoyasuda Do the design changes from https://github.com/cal-itp/mobility-marketplace/issues/500#issuecomment-1832186529 belong in this PR?

[angela-tran]

We currently don't get a preview site if the target branch is not main. A workaround for devs is to run the branch locally

[machikoyasuda]

@angela-tran No

[machikoyasuda]

@angela-tran the notes in that comment aren't actually changing anything / aren't actually reflected on Figma accurately:

• On Figma, some of the links have more spacing between them then others: 87 vs. 72

And having a hard-coded 72px number between links doesn't really make sense in a responsive web world.

• There are no styles for what the links on the Header look like on hover, and that hover style is what is setting the width of the link -- and that's how the last link in the Header gets right-aligned with the main gutter:

So until we have the Header's link styles, #501 and #502, we can't really change the nav.

cc @segacy1