Add api-docs to marketplace (#2204)

* docs: improve documentation about adding plugins to the marketplace

The relative path is misleading as one has to come from root.

* docs: add api-docs plugin to marketplace

docs/plugins/add-to-marketplace.md

@@ -6,7 +6,9 @@ title: Add to Marketplace
 ## Adding a Plugin to the Marketplace
 
 To add a new plugin to the [plugin marketplace](https://backstage.io/plugins)
-create a file in `data/plugins` with your plugin's information. Example:
+create a file in
+[`microsite/data/plugins`](https://github.com/spotify/backstage/tree/master/microsite/data/plugins)
+with your plugin's information. Example:
 
 ```yaml
 ---