Update crate guidance to match top-level guidance (#3153)

## Motivation and Context
Currently the guidance on the root repo doesn't match the guidance in
the crates themselves.
<img width="989" alt="Screenshot 2023-11-09 at 9 59 58 AM"
src="https://github.com/awslabs/smithy-rs/assets/492903/fdd11ffa-bf90-4ddc-897e-c5d5572e8c75">

## Description
Update docs to match
## Testing
- [x] post screenshot of docs once they're generated
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