Auto-generate plugin docs

Summary:
This diff changes the way on how plugin documentation is produced. Instead of keeping plugin documentation together with other docs, we will now keep it together with plugin code. There are multiple advantages of such solution:
1. We are generating docs for every plugin in a standartised way so all of them looks similar. We can also use plugin metadata for generation as well (e.g. take title, icon, oncall name etc from package.json).
2. Standartised plugin docs make it possible to build docs both for websites (public and internal) and for embedding into Flipper.
3. It will hopefully incentivise authors to write docs as they will be a part of plugin "package".
4. We can scaffold documentation template using scarf to further incentivise filling it.

Reviewed By: jknoxville

Differential Revision: D29378053

fbshipit-source-id: 66ea48dc9ba225fabfb256ae6a10f8c81eef6f5f

desktop/plugins/public/network/docs/overview.mdx

@@ -0,0 +1,7 @@
+import imgNetwork from './img/network.png';
+
+Use the Network inspector to inspect outgoing network traffic in your apps. You can easily browse all requests being made and their responses. The plugin also supports gzipped responses.
+
+<img alt="Network plugin" src={imgNetwork} />
+
+All request sent from the device will be listed in the plugin. Click on a request to see details like headers and body. You can filter the table for domain, method or status by clicking on the corresponding value in the table.