docs: Add ADR for ci on PR only

Author: Ware, Joseph (DLSLtd,RAL,LSCI)

docs/explanations/decisions/0021-ci-only-pr.md

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+# 21. Run CI on pull requests and main branch only
+
+Date: 2025-05-06
+
+## Status
+
+Accepted
+
+## Context
+
+CI has been being run on branches that do not have open pull requests, which has: added spurious jobs to the organisation's compute limit; made workflow definitions more complex; made the results page harder to parse; and allowed known working development branches to become stale or forgotten.
+
+Running CI only on branches that have open pull requests will: reduce compute jobs created for code that is known to not be working; simplify workflow definitions; simplify the CI results page; encourage early creation of pull requests, adding visibility to development.
+
+Draft pull requests will still have CI jobs run, ensuring that code quality is maintained while allowing reviewers to ignore changes that are not ready for human review.
+
+## Decision
+
+CI will only be run on all PRs when changes are made, and on the main branch when PRs are merged into it: not on branches that do not have open PRs.
+Opening a draft PR will allow CI to be run, with the understanding that the PR may not be ready for review.
+
+## Consequences
+
+The `check` job will be removed and CI will run on all PRs and on the `main` branch.