convert the ADR template to Nygard (#41428)
authored
by
When presenting the three different templates to TPA, I described them as having an increasing order of complexity, and actually said something like "you can guess I picked the most complicated one", which is counter to the spirit of the ADR adoption. We want to simplify things. So let's simplify: switch to the Nygard template, more or less, but keep the "more information" section where we can cram more things, alongside a "metadata" section that should work even without proper frontmatter support, which we're actually lacking in the GitLab wiki, but also in mdbook.