docs(proposal): create reusable artifact contribution guide

## Description

The Gemara schemas are already used to express Control Catalogs, Threat Catalogs, and Guidance Catalogs across the [FINOS CCC](https://github.com/finos/common-cloud-controls/tree/main/catalogs/core/core) & [OSPS Baseline](https://github.com/ossf/security-baseline/tree/main/baseline) projects. The procedure for proposing reusable catalogs is not documented. Therefore, it's difficult for contributors to essentially "donate" catalogs to the ecosystem because there isn't a centralized place for finding reusable catalogs.

## Expected Solution

Create a guide and set of criteria for contributors to share reusable catalogs to the Gemara ecosystem.

## Things to Consider

1. What makes a Catalog within the 7-layer architecture "reusable?"
2. What level of review must be completed to ensure the proposed Catalog(s) are accurate apart from schema-validation?
3. What is the maintenance cycle for the Catalog(s)?

## Supporting Resources

* Discussion in the community looking to donate catalogs