Are you a story teller?
So what is this one page write-up? It is a simple document that explains how it feels to use the site. In almost all the projects I have worked with, when someone new joins the team, they don't normally go through every bit of paperwork. He or she would be thrilled if they find a single-page document that quickly tells them what the site is meant to be.
You can call this document whatever you like—concept document, market requirements document, customer experience documentation, or even an Epic Fragile StoryLog™ (patent pending). It really doesn't matter.
The document should focus on the user experience rather than technical or implementation details. Make it short and interesting to read. In fact, Joel Spolsky's rule number one on documenting requirements is "Be Funny".
If possible, write about a typical user (persona in marketing speak), the problem they are facing, and how the web application solves it. Imagine how they would...