Use GitHub discussions for ADRs

[pokey]

Context

• We'd like a way to document decisions
• We use GitHub for all of our planning
• There are currently fairly few people involved in making these decisions
• ADRs are a conventional way to document decisions in a central place (see Using ADRs to document architectural decisions #1179)

Options considered

• Using GitHub discussions
• Using markdown files in docs/contributing/architecture, potentially automated by https://github.com/npryce/adr-tools

Decision

We have decided to use GitHub discussions to capture ADRs

Consequences

• Easier to discuss an ADR in perpetuity
• ADRs can be edited more easily, which is probably good for the consequences section, but bad everywhere else?
• Less permanent record of decision, as there is no PR merge / inclusion in code base
• We pollute our issue labels with labels for ADR status