Create guidance for maintenance hygiene spreadsheet

[civsiv]

Document "what good looks like" for all repos - such that new maintainers know what they should be aiming for.

• Also need maintainer docs specifying what a good contributor section looks like
• We should be able to bake this "what good looks like" markdown file into the OA GitHub org (perhaps as part of an org default GOVERNANCE.md file as per this, so that it's visible in every repo), or otherwise it could just be part of https://maintainer.openactive.io/

For criteria, please see: #3 (comment)