Document the discussion wrap-refresh process

[lubianat]

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• I would like that closed discussions that have value (Table spec proposal #64) are superseded by actionable issues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