github: add stale bot to sdg repo

[nathan-weinberg]

Same action as https://github.com/instructlab/instructlab/blob/main/.github/workflows/stale_bot.yml

[khaledsulayman]

This will be helpful. Thanks!

[bbrowning]

Approving, but also realizing this will trigger on a number of existing outstanding issues so we'll need to do a round of following up for anything we want to stay open once this lands and starts marking things as stale.