Improve Plasma Theme documentation

• Use code tags instead of italicized em tags to emphasize svg IDs.
• Cleanup network-mobile-0-[technology] blurb by adding line breaks. I don't see the -none suffix in the plasma-nm code, so I removed it and mentioned the suffix is optional.
• Fix typo in horizzontal-line.
• Fix typo/links in quickstart, and rename heading in themedetails.
• Document widgets/plasmoidheading.svg and sort the others alphabetically. Also fix formatting.
• Fix a few leftover KDEWiki {{path|...}} and <kdb> strings.
• li > code isn't styled like normal, however the pink text makes the folder names stand out more than bold text.
• Add terminal instructions to plasma theme quickstart similar to my plasma widget quick start.
• Add a quick note below describing the default and breeze-dark themes.
• Add a few example to the "Using System Colors" section as it was confusing to this user:
  https://www.reddit.com/r/kde/comments/mmosc6/is_there_any_handheld_tutorial_for_tying_a/