Explain how schema files are generated by `tauri build`

[aleclarson]

https://tauri.app/security/capabilities/#schema-files

While setting up a new Tauri project, I was confused why my src-tauri/capabilities/default.json file was pointing to a non-existent $schema. Apparently, I need to run tauri build to generate the schema? This should be documented.

[Legend-Master]

Added in #3432

[aleclarson]

Hi @Legend-Master

In #3432, I don't see any mention of the non-existent $schema path that doesn't exist until you run tauri build.

[Legend-Master]

I added a notice about they're generated by tauri-build, maybe that's not obvious enough to infer that you need to let the build script to run to get the schema files? I could include it if you think it's not that clear