docs: add docs-as-code process and one ADR per feature

docs/docs-as-code.md defines the workflow; docs/adr holds 31 ADRs
(retroactive, reconstructed from code and changelog) with a template and
index. docs/check-docs.sh validates ADR structure, index, cited code
paths, relative links and CLAUDE.md/AGENTS.md sync.

docs/README.md

@@ -0,0 +1,11 @@
+# Documentation
+
+| Document | Purpose |
+|----------|---------|
+| [docs-as-code.md](docs-as-code.md) | How documentation is written, reviewed and checked |
+| [adr/](adr/README.md) | Architecture Decision Records, one per feature |
+
+Related, at the repository root: [README](../README.md), [USERGUIDE](../USERGUIDE.md),
+[SECURITY](../SECURITY.md), [CHANGELOG](../CHANGELOG.md).
+
+Check the docs before committing: `./docs/check-docs.sh`