docs: add maintainer expectations to MAINTAINERS.md

[JoshuaKGoldberg]

PR Checklist

• Addresses an existing open issue: fixes 📝 Docs: Clarify expectations of maintainers #5513
• That issue was marked as status: accepting prs
• Steps in CONTRIBUTING.md were taken

Overview

Adds a few high-level notes on maintenance:

• It keeps the existing casual tone that this isn't a corporate role. Folks can contribute on their own schedule.
• Adds a expected average activity levels: a few contributions of any kind a week, and one non-trivial one a week
  • Explicitly calls out that these are averages over time, not expected for every week
  • Also notes allowances for skipping weeks and even months
• Establishes timeframes for inactivity, leaving maintenance, and coming back
  • To make this timeline more clear, I added a note on taking a month for important decisions (which I think implicitly includes clearing inactive maintainers)