Document "check" events and metrics

[BigLep]

Done Criteria

There is a public document that lists

• The set of "events" that occur within the various "dealbot" checks (e.g., "Data Storage", "Retrieval") including the event definition and source of truth
• Metrics that are computed from these checks

Why Important

1. Drives clarity for engineering team on what to implement
2. Gives a canonical resource for the dealbot dashboard to link to so users can know what a metric means and how it's computer

User/Customer

1. Maintainers
2. Dashboard consumers who have questions about what metrics mean

Notes

1. Markdown file in the repo is fine.
2. Explorative drafting has been happening in https://www.notion.so/filecoindev/dealbot-metric-scratchpad-2f1dc41950c1808b94e1fe85babc4f00

[BigLep]

#228 will merge into #164 which will address this issue