Improve Project Hierarchy Description

[DavidMansolino]

This page should be completed/improved: https://www.cyberbotics.com/doc/guide/the-standard-file-hierarchy-of-a-project?version=develop

We should at least add:

• the libraries folder.
• the plugins/remote_controls folder.
• the plugins/robot_windows folder.
• the font, sound, etc. folders
• the protos/textures and protos/icons folder
• Etc.

We should also add a diagram showing a standard file hierarchy.