[Proposal] Improve Contributor Ladder and Role Definitions

## Objective

Currently, our governance describes a flat maintainer structure. We need to formalize how contributors advance and define the maintainer split between the subprojects.

## Proposed Ladder

- Sustained Contributor: Active community member submitting PRs/Issues.
- Subproject Maintainer: PR approval and write access for a specific repository or collection of repositories.
- Core Maintainer: Organization-wide oversight, spec maintainers, and binding votes on project governance.
- Community Manager: Neutral facilitator for outreach and moderation and documentation maintenance.

## Proposed Completion Criteria

- [x] Define the specific expectations and "rung" requirements for each role in the ladder and the process to move to the "rung".
- [x] Amend the existing Governance doc to distinguish between "Core" and "Subproject" maintainer roles.
- [ ] Standardize how we list people across repos so their role is clear (Can we just stick to `security-insights`?)
- [x] Create GitHub Team Mapping and Implementation
- [x] Implement CODEOWNERS to automatically request reviews from the correct Subproject team.
- [ ] Reflect the new contributor ladder on the website's Community page.