Add documentation for no_metadata tag

[Jacalz]

This was added in v2.5.0 but not added to the docs. Adding it now and updating my apps to use it.

[andydotxyz]

Good catch thanks.
However I struggled to parse the sentence - it kindof assumes that people know runtime metadata lookup is off for release - I wonder if that part isn't hugely relevant? If it is can it be expressed in a way that is more explicit?

[Jacalz]

I kind of struggled with how to write it or be honest. Saying that it disables all metadata would be wrong as anything bundled by cmd/fyne will still work, I think.

[andydotxyz]

How about "Disable runtime lookup of metadata from FyneApp.toml source files. This is always disabled for release builds."

[Jacalz]

Sounds splendid :)