chore: add doc for martkeplace (#4395)

Summary:
Provide a documentation about marketplace feature of Flipper.

This should cover:
1. Introduction of the feature (plugin discovery, auto update)2.
2. Marketplace server - to list available plugins
3. Flipper settings - describe the new settings and how to enable4.

Closes https://github.com/facebook/flipper/issues/3545.

## Changelog

Pull Request resolved: https://github.com/facebook/flipper/pull/4395

Reviewed By: antonk52

Differential Revision: D42918936

Pulled By: passy

fbshipit-source-id: 50b10178b569ecc6ea65b736ea58db401cf686c6

website/sidebars.js

@@ -144,6 +144,7 @@ module.exports = {
           'extending/layout-inspector',
         ],
       },
+
     ],
     'Under the Hood': [
       'internals/index',