[Request]: Document mapping of integration variable types to EUI fields, other Fleet/Integrations UI concerns

### Description

Fleet is responsible for generating a "policy editor" from integration manifests that allows for user input on integration policies. As part of this process, Fleet essentially translates the integration's various `manifest.yml` file into a dynamic `EuiForm` component that allows user input for all the integration's defined variables.

Today, we don't really have this process documented, and integration maintainers aren't necessarily familiar enough with EUI to make proper decisions about which variable type to use, what type of content to include for help text, labels, descriptions, etc. We should close this gap.

### Resources

[Fleet dev docs that touch on this a bit](https://github.com/elastic/kibana/blob/main/x-pack/plugins/fleet/dev_docs/integrations_overview.md)

### Which documentation set does this change impact?

Integrations

### Feature differences

N/A

### What release is this request related to?

N/A

### Collaboration model

The engineering team

### Point of contact.

**Main contact:** @kpollich

**Stakeholders:** @elastic/ecosystem