docs: add GOVERNANCE.md

[najuna-brian]

Adds GOVERNANCE.md, the written version of the ODE Governance Framework v1.0 voted in on 19 Sep 2026. Because it lives in the organisation's .github repo, GitHub shows it across all ODE repositories.

Section	What it covers
Principles	Eligibility through merit, selection through voting
Roles	Community member, maintainer, council member, council lead, General Secretary
Councils	The five councils and their responsibilities, 5 seats each
Seats	Which councils currently have seats available
Becoming a council member	Eligibility, application, review, Governance Council vote, 2-year term
Transparency	Minutes are public; decisions are announced on the forum
Changing this document	Changes by pull request and a Governance Council vote

Note for reviewers: "Changing this document" is not in the Framework v1.0 text. Please confirm or remove it.

Related: the docs pages in OpenDataEnsemble/ode link to this file.