[Recommended] Enhance Governance documentation

[kfaseela]

Hello OpenFGA Community,

As part of OpenFGA Incubation request, the CNCF TOC has noted down some improvement suggestions that would be beneficial for the OpenFGA community in the long term and would eventually be useful at the time of Graduation.

As part of the Incubation review, the documentation was considerably expanded to cover governance and maintainer details in GOVERNANCE.md and MAINTAINERS.md, ensuring clarity in contributor pathways and decision-making.
However, there is still scope for improvement. It would be nice to document:

1. clearer processes for roadmap approvals
2. leadership decisions and elections
3. potential subproject governance.
4. leadership roles, and better separation between upstream/downstream

[kfaseela]

Would be great if GOVERNANCE.md can be moved to the community repo and then linked from other repos. MAINTAINERS.md currently resides in community repo. Would be nice to have it uniform.