Convert internal links in HIG to relref shortcodes

This covers the bulk of links inside the hig (and uncovered some incorrect links already). Some issues remain:

• content/hig/components/navigation/actionbutton.md:58
  For some reason the relref inside a shortcode block doesn't work as expected. The page name itself is ambiguous, but when I use the full path, hugo doesn't like it either.
• content/hig/style/writing/localization.md:29
  There is a stale link (KDE_Visual_Design_Group/HIG/Keyboard_Shortcuts) that doesn't seem to have any equivalent page
• content/hig/platform/settings.md:53
  The link text "design mantra" (in the context of "Simple by default, powerful when needed") links to the platform page. That seems odd.
• content/hig/style/color/dark.md, content/hig/style/color/default.md, content/hig/style/color/high.md, content/hig/style/color/light.md
  On these pages, the following sentence could possibly be better written. I didn't fix the link.

  See here for more information about usage of colors.