Move plugins to "sonar/desktop/plugins"

Summary:
Plugins moved from "sonar/desktop/src/plugins" to "sonar/desktop/plugins".

Fixed all the paths after moving.

New "desktop" folder structure:
- `src` - Flipper desktop app JS code executing in Electron Renderer (Chrome) process.
- `static` - Flipper desktop app JS code executing in Electron Main (Node.js) process.
- `plugins` - Flipper desktop JS plugins.
- `pkg` - Flipper packaging lib and CLI tool.
- `doctor` - Flipper diagnostics lib and CLI tool.
- `scripts` - Build scripts for Flipper desktop app.
- `headless` - Headless version of Flipper desktop app.
- `headless-tests` - Integration tests running agains Flipper headless version.

Reviewed By: mweststrate

Differential Revision: D20344186

fbshipit-source-id: d020da970b2ea1e001f9061a8782bfeb54e31ba0

docs/tutorial/js-table.md

@@ -129,7 +129,7 @@ export default createTablePlugin<Row>({
   buildRow,
 });
 ```
-*See [index.tsx](https://github.com/facebook/flipper/blob/master/desktop/src/plugins/seamammals/index.tsx)*
+*See [index.tsx](https://github.com/facebook/flipper/blob/master/desktop/plugins/seamammals/index.tsx)*
 
 The `method` we define here corresponds to the name
 of the function we call on the native side to inform