Update Eng-Product joint OKR guidance

This MR updates guidance to better outline the requirements on the Engineering side for joint OKRs with Product.

To try not to duplicate too much information, the Engineering part of the handbook has minimal information, pointing to the Product page for details.

Extracts from Slack discussion:

Farnoosh 4 hours ago To Christopher's point, CEO doesn't firm up OKRs till about 8 weeks prior to quarter start according to that page. And in my experience they don't firm up till about 5 weeks prior to new quarter start. So it's not till around 4 weeks prior to quarter start the the Product Division > will have drafts and be in conversation with Engineering on shared KRs / collaboration.

Arty-chan 4 hours ago I imagine the idea behind the handbook guidance that I quoted is for Engineering to start the discussion with Product on possible shared OKRs and potentially draft them, not necessarily entirely firmed up.

leif 4 hours ago Okay. I think the section in the engineering part needs to better align.

Farnoosh 3 hours ago This is what I'm thinking that's not our product OKR page yet, so we can disucss at 6 weeks - Engineering reviews any proposed shared OKRs with Product (KRs we need you to help us do)

at 4 weeks -- Engineering confirms alignment to shared OKRs