Dedicated page for inter-plugin communication

[boformer]

Right now there is no explanation how to communicate with other plugins. The basic information is there, but "interoperability" is not mentioned.

The text could also be a section of the "best practices" page.

Topics (with link to other pages):

• @Plugin dependency string
• Soft dependencies
• Services
• Custom events
• Event order
• Plugins with multiple modules (child plugins)

More ideas?