Clean up Qt for Embedded Linux to improve readability

The Qt for Embedded Linux page has a lot of useful information, but currently, it's not well-structured. Here's what I would like to do:

• Split the topics into smaller pages to improve readability and make the content more digestible
• Use tables for the environment variables, like the change that was done recently for the EGLFS section, so that the content can be skimmed
• "Input" and "Display Output" could also be pulled out as separate pages
• Eventually, the main page should be a short overview and point developers to smaller sub-pages so that they can quickly find what they are looking for