docs: add architecture documentation and slides

docs/README.md

@@ -0,0 +1,21 @@
+# Documentation
+
+This directory contains the design notes, diagrams, and presentation material for the Gapido backend code challenge.
+
+## Contents
+
+- [Architecture](architecture.md): service boundaries, flows, deployment modes, and runtime topology.
+- [Implementation Decisions](implementation-decisions.md): key decisions and tradeoffs made during development.
+- [ADR Index](adr/README.md): focused architectural decision records.
+- [Slides](slides/index.html): static HTML presentation deck.
+- [Diagrams](assets/diagrams/): generated PNG diagrams used by docs and slides.
+
+## Regenerate Diagrams
+
+Graphviz must be installed on the machine because the Python `diagrams` package renders through Graphviz.
+
+```bash
+pip install -e ".[dev]"
+python docs/diagrams/generate.py
+```
+