Add demo and share new URL (#356)

# Description

This PR adds a DEMO section for the safe app

The goal is to allow to get a feel of the features of the dapp before
you follow the tutorial.

I created a new URL for this demo app in here
https://safe.widget.cow.fi/

---------

Co-authored-by: mfw78 <[email protected]>

docs/cow-protocol/tutorials/swap-in-safe-app/README.mdx

@@ -13,6 +13,22 @@ This is interesting for Safe Dapps because:
 
 The easiest way to get started is to follow the steps below.
 
+## Demo
+Before you start, you can get a feel of the Safe App example. Feel free to skip this step!
+
+The example shows:
+- How to make a new Safe App
+- How to make Swaps, Limit orders and TWAPs
+- How to control programatically the swap widget
+- How to detect trade parameters (token selection, amounts, etc.)
+- How to define a dynamic fee
+- How to customize the colors, tokens, sounds, etc
+
+
+To use the demo, just follow the instructions in [Add your custom dapp in Safe Wallet](#add-custom-dapp) below, but instead of adding the URL `http://localhost:9999`, use the URL `https://safe.widget.cow.fi`.
+
+![Safe custom dapps 0](/img/safe-app/safe-custom-apps-0.png)
+
 ## Fork the project
 You can use the **safe-app-example** to start with some basic boilerplate code.
 
@@ -40,7 +56,7 @@ PORT=9999 yarn start
 
 The app should be running on http://localhost:9999.
 
-## Add your custom dapp in Safe Wallet
+## Add your custom dapp in Safe Wallet {#add-custom-dapp}
 Go to https://app.safe.global and create a safe if you don't have one for Sepolia network (convenient for testing).