[docs] Adding more doc examples for working with file system on android/ios

Hi,

in my company we had big struggles because we wanted to show <img src={"path/in/appdata/image.png"} /> it took incredibly a lot of effort to find the way how to do it. With some previous discussions on discord for my windows app we were able to patch some working code.

The magic how we found solution:

1. if I wasn't asking this on discord long time ago my colleague would never find in documentation convertFileSrc
2. after finding the function, we could not find proper security settings, we digged it from some forum.

Please add functionality convertFileSrc and example here:

https://v2.tauri.app/plugin/file-system/

It's quite logic, when I work with file system I want to show maybe some image related to data. Also some libs work with url path where I can't do readFile()

Also in https://v2.tauri.app/reference/javascript/api/namespacecore/#convertfilesrc I can't see tauri.config which enables getting the file....

Start with the file-system plugin page at https://v2.tauri.app/plugin/file-system/ and the convertFileSrc reference at https://v2.tauri.app/reference/javascript/api/namespacecore/#convertfilesrc. Document how to use convertFileSrc for an image path and include the required security configuration; done means both pages clearly explain the working setup.