Improve QmlLive documentation

Review the QmlLive documentation to look for ways to improve the content so that it is more accurate and consistent across other Qt Automotive Suite documentation.

• Define more terms in the Terminology list
• Remove noise and simplify the documentation so that it is more readable, especially by non-native English speakers
• Update the screenshots, especially concepts.qdoc that's missing a Sample Sheet screenshot (at the end)
• Change the screenshot in https://doc.qt.io/QtQmlLive/qmllive-contentplugin-example.html to use something better, that's more professional.