Any document regarding how to save apigee code in stash.
What exactly is "stash"?
Apigee configuration for API proxies is just XML files. In fact you can download any proxy as a "bundle" - which is just a zip file containing a bunch of XML files and resource files. You can store the bundle anywhere, either exploded or not.
Other configuration - like that for API products, or KVM, or vhosts etc... can also be stored as XML or JSON. They're just files. There is nothing special about them.
I assume you mean Atlassian Stash which is now known as Bitbucket Server and is a popular Git Repository.
As Dino mentioned, Apigee code are just configuration files. You would store them into a SCM like any other code.
What is the complication you are running into?