Create guide: How to use ytt with Helm

## Why
Authors or consumers can use helm and ytt together, but they may believe that it's not possible since we have no documentation on how to do it or what use cases it would serve. These guides are intended to be lightweight and highlight these use cases with simple examples.
## What
Some use cases we can touch on are:
- Patching Helm templates: show example of Helm templates with a ytt overlay Helm post-renderer. (ie: --post-renderer "./overlays.sh)
- There's an example of this [here](https://github.com/vmware-tanzu/carvel-ytt/pull/572/files#diff-37d3fcc528cc1da3c4985a4bdc1049c091edb419166aba05904e96b972a3b812)
- Schema type checking Helm inputs: Using ytt Schemas to type check values that are then piped into helm templates. (ie: ytt -f schema.yml -f values.yml | helm upgrade –values -)
- ...
## How
In a page the carvel.dev/ytt documentation