docs: restructure docs (#14421)

Closes #13434 
Supersedes #14182

---------

Co-authored-by: Ethan <39577870+ethanndickson@users.noreply.github.com>
Co-authored-by: Ethan Dickson <ethan@coder.com>
Co-authored-by: Ben Potter <ben@coder.com>
Co-authored-by: Stephen Kirby <58410745+stirby@users.noreply.github.com>
Co-authored-by: Stephen Kirby <me@skirby.dev>
Co-authored-by: EdwardAngert <17991901+EdwardAngert@users.noreply.github.com>
Co-authored-by: Edward Angert <EdwardAngert@users.noreply.github.com>

docs/admin/infrastructure/index.md

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+# Infrastructure
+
+Learn how to spin up & manage Coder infrastructure.
+
+## Architecture
+
+Coder is a self-hosted platform that runs on your own servers. For large
+deployments, we recommend running the control plane on Kubernetes. Workspaces
+can be run as VMs or Kubernetes pods. The control plane (`coderd`) runs in a
+single region. However, workspace proxies, provisioners, and workspaces can run
+across regions or even cloud providers for the optimal developer experience.
+
+Learn more about Coder's
+[architecture, concepts, and dependencies](./architecture.md).
+
+## Reference Architectures
+
+We publish [reference architectures](./validated-architectures/index.md) that
+include best practices around Coder configuration, infrastructure sizing,
+autoscaling, and operational readiness for different deployment sizes (e.g.
+`Up to 2000 users`).
+
+## Scale Tests
+
+Use our [scale test utility](./scale-utility.md) that can be run on your Coder
+deployment to simulate user activity and measure performance.
+
+## Monitoring
+
+See our dedicated [Monitoring](../monitoring/index.md) section for details
+around monitoring your Coder deployment via a bundled Grafana dashboard, health
+check, and/or within your own observability stack via Prometheus metrics.