Upgrade static docs plugin

Summary:
Upgrade the static docs plugin.

You no longer need to import OssOnly and FbInternalOnly in markdown files, so removed those imports too.

Reviewed By: passy

Differential Revision: D26580059

fbshipit-source-id: 2763de2f5fbef41ec2ac7f7bdd147418badb78b6

docs/extending/flipper-plugin.mdx

@@ -2,7 +2,6 @@
 id: flipper-plugin
 title: Desktop Plugin API
 ---
-import {FbInternalOnly} from 'internaldocs-fb-helpers';
 
 ## PluginClient