Document the discussion wrap-refresh process

Many PRs and issues around here contain valuable discussions that may drive the future of the NGFF spec.

However, they often get stale, as they depended on something to get done. Some get forgotten, some get transformed and some keep very relevant through time.

It would be good to have a process for wrapping an old, staled discussion and re-writing it as a new Issue, perhaps with a linked PR.

It could be called a wrap-refresh process, with some structure on how to do it in a sensible manner.

User stories

As someone triaging tickets on NGFF...

• I would like to be able to determine if an issue/PR is still relevant and should be actively tracked.
• I would like that closed discussions that have value (https://github.com/ome/ngff/pull/64) are superseded by actionable issues

As a new-ish contributor to NGFF...

• I would like to see summaries of open ideas/issues/discussions without having to dive head-first in lengthy discussions

Start by reviewing the discussion patterns described in this issue and the closed discussion in https://github.com/ome/ngff/pull/64. Define a documented wrap-refresh process for assessing stale issues and PRs, summarizing valuable discussions, and creating actionable superseding issues. Done means the process is clear enough for triagers and new contributors to follow.