Add documentation and documentation testing

docs/src/getLedgerVersion.md.ejs

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+## getLedgerVersion
+
+`getLedgerVersion(): Promise<number>`
+
+Returns the most recent validated ledger version number known to the connected server.
+
+### Parameters
+
+This method has no parameters.
+
+### Return Value
+
+This method returns a promise that resolves with a positive integer representing the most recent validated ledger version number known to the connected server.
+
+### Example
+
+```javascript
+return api.getLedgerVersion().then(ledgerVersion => {
+  /* ... */
+});
+```
+
+```json
+16869039
+```
+