Adopt ADR documents

Jasonelle documents decisions implicitly across issues, changelogs, and source code, which makes it hard to know why a design choice was made. We want a lightweight, versioned way to record important decisions and their rationale so future contributors can understand the reasoning.

Chosen option: "Use Architecture Decision Records (ADRs) following the MADR format", because ADRs are lightweight, versioned alongside the code in this repository, and published automatically with the Antora docs. Positive consequences: decisions are easy to find, reviewable in pull requests, and searchable in the published site. Negative consequences: a small authoring overhead is required to keep records up to date.

ADR is the best option, because the other options scatter knowledge outside the codebase or outside review. If "Keep documenting decisions ad hoc in issue comments" was chosen, decisions would remain hard to discover. If "Use a standalone wiki" was chosen, the wiki would drift out of sync with the code.