Update the proposals process

[masci]

Proposed Changes:

• Clarify the process and the people involved in the process.
• Add a Github workflow to manage stale proposals

[mayankjobanputra]

@masci LGTM.

I have maybe one question regarding changes that require a proposal:

1. What about decisions like setting flags (i.e., TORCH_FORCE_WEIGHTS_ONLY_LOAD = True) by default for Haystack? Do we need proposals for such decisions?

[julian-risch]

Looks good to me. I especially like that it's crystal clear now when the "final comment" period starts.
One small detail, where we could be more clear is in the phrase "we add a comment and a label to your PR.". We could write down explicitly who is "we" here. It should be the decision driver in my opinion. An alternative would be the third core contributor who approves the proposal but that's easier to miss.