usage-data: generate markdown list of all allowed events in build

Created by: coury-clark

Events will be restricted from scrape based on an allowlist, stored in a table. This makes viewing the total set of the allowlist somewhat difficult. This issue is a "nice to have", but it would be good to generate a markdown file containing a list of all of the allowed events, similar to how sg migration describe produces schema documentation.

https://github.com/sourcegraph/sourcegraph/issues/39094#issuecomment-1197271769_