Reorganize Plasma section

What does this merge request add?

• Fixes a few links
• Mentions Splash Screen
• Changes from lists to headings + descriptions + lists
• Puts global theme at the top and links to its components

Screenshot of the result (optional)

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How to test this MR

git clone https://invent.kde.org/documentation/develop-kde-org.git
cd develop-kde-org
git mr 285
hugo server