Add API overview page for Web Midi API

[wbamberg]

GroupData.json lists "Web Midi API" as a Web API with the following contents:

        "Web MIDI API": {
            "interfaces": [ "MIDIInputMap",
                            "MIDIOutputMap",
                            "MIDIAccess",
                            "MIDIPort",
                            "MIDIInput",
                            "MIDIOutput",
                            "MIDIMessageEvent",
                            "MIDIConnectionEvent" ],
            "methods":    [ "Navigator.requestMIDIAccess()"],
            "properties": [],
            "events":     []
        },

...but it doesn't have an API overview page. We should write a page and add the "overview" property to GroupData.

[teoli2003]

For the record, @rachelandrew created the overview page in #5433.