docs: add repository deleting data page (#28)

Author: kiaking

docs/guide/repository/deleting-data.md

@@ -1,3 +1,74 @@
 # Repository: Deleting Data
 
-Coming soon...
+You may delete existing data through various repository methods.
+
+In this section, it assumes you're familiar with the usage of repository. If not, please read through the [Repository: Getting Started](./getting-started) page first.
+
+## Deleting Data
+
+To delete a record, you may call `destroy` method and pass the primary key for the record.
+
+```js
+// Existing records.
+[
+  { id: 1, name: 'John Doe', age: 40 },
+  { id: 2, name: 'Jane Doe', age: 30 },
+  { id: 3, name: 'Johnny Doe', age: 20 }
+]
+
+// Delete the reocrd with id of 2.
+store.$repo(User).destroy(2)
+
+// The result.
+[
+  { id: 1, name: 'John Doe', age: 40 },
+  { id: 3, name: 'Johnny Doe', age: 20 }
+]
+```
+
+In addition to a single primary key as its argument, the `destroy` method will accept an array of primary keys to delete multiple records.
+
+```js
+store.$repo(User).destroy([1, 2])
+```
+
+The `destroy` method will return deleted models. When you pass a single primary key, it will return a single model, and if you pass multiple primary keys, it will return a collection of models.
+
+```js
+const user = await store.$repo(User).destroy(2)
+
+// { id: 2, name: 'Jane Doe', age: 30 }
+
+const user = await store.$repo(User).destroy([1, 2])
+
+/*
+  [
+    { id: 1, name: 'John Doe', age: 40 },
+    { id: 2, name: 'Jane Doe', age: 30 }
+  ]
+*/
+```
+
+If you wish to delete the entire records, you may use the `flush` method.
+
+```js
+store.$repo(User).flush()
+```
+
+## Deleting data by query
+
+You can also run a delete statement on a set of records. In this example, we will delete all flights that are marked as inactive.
+
+```js
+store.$repo(User).where('active', false).delete()
+```
+
+## Deleting data by model instance method
+
+You may delete a specific record with the model `$delete` instance method.
+
+```js
+const user = store.$repo(User).find(1)
+
+user.$delete()
+```