[docs] Add more detail to unsupported platforms

In `[package.metadata.platforms]` there are a number of things that are marked `"none"`. For example in `tauri-plugin-sql` for `ios`:

https://github.com/tauri-apps/plugins-workspace/blob/e8069af80401cfc1e90297b786bef05a929ed9a6/plugins/sql/Cargo.toml#L17-L22

I just went the Tauri Discord server to go ask for more info on why it's not supported. (Is it planned? Is there something blocking support? etc)

It would be nice (at least on the docs site) if unsupported platforms were a bit more broken down, either with more `"level"`'s or just more notes being added.

I think this list would be exhaustive:

- **Inapplicable:** Does not apply to this platform (ex: `tauri-plugin-cli` on iOS/Android)
- **Unplanned:** Have not yet seen enough interest in supporting, or just not high enough priority to have been looked into, but it hasn't been rejected
- **Planned:** Maintainers want to support this platform, but work hasn't started.
- **_For Note:_** If it's on the roadmap for a specific future release
- **Blocked:** Blocked on missing platform APIs or third-party deps don't support the platform.
- **_For Note:_** Details on what is blocking support for the platform.
- **Wontfix:** Maintainers do not want to support the plugin on this platform (ex: Maintenance cost)
- These two are more workflow things that might be weird to put in the `Cargo.toml` but useful to know:
- **In Development:** Some work has been done but it's not enabled yet
- **In Preview:** Available in pre-release version, but not yet in a stable release