Docs about the virtual file system

[abose]

We need to have a section on the virual file system used by phoenix code.
the main APIs Phoenix.fs.getTauriPlatformPath, Phoenix.VFS.getTauriVirtualPath, Phoenix.app.getDisplayPath, Phoenix.app.getDisplayLocation etc..

[abose]

https://github.com/phcode-dev/phoenix-fs see readme.md, the below two sections could be adapted somehow without going too much into the implementation details:

1. Phoenix Browser Virtual File System
2. File System Structure & Organization