Document cells 1.0 will not have new customers

What does this MR do and why?

What

Update the Cell 1.0 iteration to specify we will not onboard new customers.

Why

After discussion during Cells Fast boot, we came to the decision that Cells 1.0 will only be for internal customers for:

1. Achieve the timeline for FY25-Q4 deployment.
2. We don't have a specific customer in mind that is willing to onboard with some features cut.
3. Not having a customer in mind makes it hard to figure out what scope to cut.
4. We can start with internal customers like finance, and security (examples, not decisions) to dogfood earlier.