fix(docs): Add how to reset state to docs/guides section (#1236)

* Add how to reset state to docs/guides section

* Fixing code examples

* Minor changes

* Minor fixes

* Adding codesandbox demo

* Update codesandbox demos

docs/guides/how-to-reset-state.md

@@ -0,0 +1,80 @@
+---
+title: How to reset state
+nav: 18
+---
+
+The following pattern can be used to reset the state to its initial value.
+
+```ts
+import create from 'zustand'
+
+// define types for state values and actions separately
+type State = {
+  salmon: number
+  tuna: number
+}
+
+type Actions = {
+  addSalmon: (qty: number) => void
+  addTuna: (qty: number) => void
+  reset: () => void
+}
+
+// define the initial state
+const initialState: State = {
+  salmon: 0,
+  tuna: 0,
+}
+
+// create store
+const useSlice = create<State & Actions>((set, get) => ({
+  ...initialState,
+
+  addSalmon: (qty: number) => {
+    set({ salmon: get().salmon + qty })
+  },
+
+  addTuna: (qty: number) => {
+    set({ tuna: get().tuna + qty })
+  },
+
+  reset: () => {
+    set(initialState)
+  },
+}))
+```
+
+Resetting multiple stores at once instead of individual stores
+
+```ts
+import _create, { StateCreator, StoreApi, UseBoundStore } from 'zustand'
+
+const resetters: (() => void)[] = []
+
+export const create = <TState extends unknown>(
+  createState: StateCreator<TState> | StoreApi<TState>
+): UseBoundStore<StoreApi<TState>> => {
+  // We need to use createState as never to support StateCreator<TState> and
+  // StoreApi<TState> at the same time.
+  // We also need to re-type slice to UseBoundStore<StoreApi<TState>>
+  const slice: UseBoundStore<StoreApi<TState>> = _create(createState as never)
+  const initialState = slice.getState()
+
+  resetters.push(() => {
+    slice.setState(initialState, true)
+  })
+
+  return slice
+}
+
+export const resetAllSlices = () => {
+  for (const resetter of resetters) {
+    resetter()
+  }
+}
+```
+
+## CodeSandbox Demo
+
+- Basic: https://codesandbox.io/s/zustand-how-to-reset-state-basic-demo-rrqyon
+- Advanced: https://codesandbox.io/s/zustand-how-to-reset-state-advanced-demo-gtu0qe