Create documentation aimed at "template authors"

[tsandall]

We need docs that explain how people can extend Gatekeeper with their own templates. For example, we need to explain:

• Deny rule semantics
• The structure of the input document
• How to use JSON Schema to define constraint parameter schema
• How to write policies that depend on inventory
• Difference between the raw Rego and template kinds
• How to upstream templates
• How build-to-yaml process works
• How to debug Rego when constraints aren't matching

[ritazh]

We need this documentation to cover opa and vap in the future.

[sozercan]

https://github.com/open-policy-agent/frameworks/tree/master/constraint can be used as base (except target)

[salaxander]

Looks like documentation on accessing values from data.inventory is here -> https://open-policy-agent.github.io/gatekeeper/website/docs/sync/

[salaxander]

• [ ] Deny rule semantics
• [x] The structure of the input document
• [ ] How to use JSON Schema to define constraint parameter schema
• [x] How to write policies that depend on inventory
• [x] Difference between the raw Rego and template kinds
• [x] How to upstream templates
• [ ] How build-to-yaml process works
• [ ] How to debug Rego when constraints aren't matching