[Recommended] Provide clarity on contributor ladder

[kfaseela]

Hello OpenFGA Community,

While OpenFGA defines multiple contributor roles in its governance documentation, including core, product, maintainers, and contributors, with responsibilities outlined in GOVERNANCE.md, it is not clear on how a contributor can grow to a maintainer or core member. It would be nice to enhance the documentation to clearly show the contributor ladder and the eligibility criteria and process to move levels.