Document the new deeplink format [2/n]

Summary:
Added docs on how to use deeplink.

Note that not all features are implemented yet.

Reviewed By: fabiomassimo

Differential Revision: D29845044

fbshipit-source-id: 835fba8ce97c2106d160f347675e11d1ed749fba

docs/extending/deeplinks.mdx

@@ -0,0 +1,34 @@
+---
+id: deeplinks
+title: Using deeplinks
+---
+
+Flipper supports opening Flipper through deeplinks, by using the `flipper://` protocol. 
+
+## open-plugin
+
+The follow link format can be used to open Flipper, and open a specific plugin:
+
+`flipper://open-plugin?plugin-id=<plugin-id>&client=<client>&devices=<devices>&payload=<payload>`
+
+The parameters are specified as follows:
+
+* `plugin-id` (required) the identifier of the plugin that needs to be opened, as specified by the `id` field in `package.json`.
+* `client` (optional) the name of the application that should be opened. For device plugins, this doesn't need to be specified. If not set, the user will be prompted to select a client.
+* `devices` (optional) comma separated list of device types that are acceptable. E.g. `iOS,Android` or `Metro`. If set, `client` and `plugin-id` must be running on this type of device.
+* `payload` (optional) any additional string argument to pass to the plugin. Note that this argument should be properly URL encoded.   
+
+
+Using this deeplink format will make sure that:
+
+* [FB-ONLY] The user is logged in & connected to Facebook.
+* Verify that Flipper is up to date.
+* [FB-ONLY] Install the specified plugin if not installed already.
+* Make sure the plugin is available on the provided client / device, or will prompt the user for a valid device / client selection is made on which the plugin is available.
+* Enable the plugin if not enabled already.
+
+### Handling deeplinks in the plugin
+
+If a plugin is opened through a deeplink, for which a `payload` was set, the [`onDeepLink`](flipper-plugin#ondeeplink) handler will be triggered directly after initializing and rendering the plugin.
+
+Note that the same payload format can also be used to open other plugins programmatically from inside another plugin, by passing the payload as second argument to [`selectPlugin`](flipper-plugin#selectplugin).

website/sidebars.js

@@ -105,6 +105,7 @@ module.exports = {
     'Desktop plugin APIs': [
       'extending/flipper-plugin',
       'extending/styling-components',
+      'extending/deeplinks',
       ...fbInternalOnly([
         {
           'QPL linting': ['fb/building-a-linter', 'fb/active-linters'],