Add more information about Services used and their pricing models to readme.

Jeremy provided the following context around the Readme which I think makes sense. I don't think I'll have the time to add it though, so I wanted to capture it as a separate work item. Here's his comment:

> I would kind of like a section near the top (maybe part of the first section) that lays clearly the services (and their purpose) and mentions pricing model if I want to take this project to production. It doesn’t have to provide exact pricing (and probably shouldn’t) but it couldshould probably disclose the pricing model for each service. Ie, availability of free tiers, priced by teams or usage or both, etc. f we could provide example pricing we’ve seen among clients (ie 1mil req day ~= $100 / mo) that would be super useful. Example (I made these up). I
> 1. Fastly is used for content delivery, it makes the delivery of your app to all regions super fast. Its priced w/ a free tier, and then by usage. See [link to pricing page] for more details.
> 2. Vercel is the service we use to actually deploy the app (think like managed AWS). Priced by team size with generous quotas and priced for overages. See [link to pricing page] for more details.
> 3. Sanity is your content management utility. It’s priced by team size (for managing content) and by usage. See [link to pricing page] for more details.

Start in the repository README and review how Fastly, Vercel, and Sanity are currently described. Add a section explaining each service's purpose, pricing model, free tiers or quotas, overage or usage costs, and links to current pricing pages; completion means all three services are covered without promising exact prices.

Written by the indexing model from the issue text.