Add documentation and documentation testing

docs/src/getFee.md.ejs

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+## getFee
+
+`getFee(): Promise<number>`
+
+Returns the estimated transaction fee for the server(s) the RippleAPI instance is connected to.
+
+### Parameters
+
+This method has no parameters.
+
+### Return Value
+
+This method returns a promise that resolves with a floating point value representing the estimated fee to submit a transaction, expressed in XRP.
+
+### Example
+
+```javascript
+return api.getFee().then(fee => {/* ... */});
+```
+
+```json
+0.012
+```