Move Single Asset Vault docs from opensource

docs/references/http-websocket-apis/public-api-methods/vault-methods/index.md

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+---
+metadata:
+  indexPage: true
+---
+# Vault Methods
+
+A `Vault` object in the XRP Ledger represents the state of a tokenized vault. Use these methods to interact with vaults.
+
+{% child-pages /%}