Split out content to many smaller files

And add markmaker.py to generate workshop.md

docs/security.md

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+# Secrets management and encryption at rest
+
+(New in Docker Engine 1.13)
+
+- Secrets management = selectively and securely bring secrets to services
+
+- Encryption at rest = protect against storage theft or prying
+
+- Remember:
+
+  - control plane is authenticated through mutual TLS, certs rotated every 90 days
+
+  - control plane is encrypted with AES-GCM, keys rotated every 12 hours
+
+  - data plane is not encrypted by default (for performance reasons),
+    <br/>but we saw earlier how to enable that with a single flag