docs(security): add collateralization ratio docs

Author: albie

documentation/properties/collateralization_ratio.md

@@ -0,0 +1,16 @@
+---
+layout: property
+title: "collateralization_ratio"
+schemas: [security]
+---
+
+# collateralization_ratio
+
+---
+
+Collateralization compares the value of cover pool assets to the value of covered bonds, on
+nominal or net present value basis, depending on the credit rating agency and the issuer. Committed
+overcollateralization is the overcollateralization that the issuer is required to maintain, either based
+on the covered bond legal framework or the contractual obligation. Unit: % (of covered bonds).
+
+See [ECB Working Paper No 2393](https://www.ecb.europa.eu/pub/pdf/scpwps/ecb.wp2393~f0796c2019.en.pdf)