Add "Plugins" page to the website navigation panel (#187)

The project documentation is published in a website generated by MkDocs.

The primary UI for discovering and accessing the various pages of the site is through the navigation panel/menu. In order to have control over the structure, the contents of the navigation element are explicitly defined in the `nav` key of the MkDocs configuration file.

A new "Plugins" page was added to the documentation. Previously that page it was not included in the `nav` key so the page was not listed in the website's navigation element.

Author: per1234

mkdocs.yml

@@ -59,3 +59,4 @@ nav:
       - firmware get-version: commands/arduino-fwuploader_firmware_get-version.md
       - firmware list: commands/arduino-fwuploader_firmware_list.md
       - version: commands/arduino-fwuploader_version.md
+  - Plugins: plugins.md