This is a new HIG that I've written to replace the old one.

Closes https://invent.kde.org/teams/vdg/issues/-/issues/10

Closes https://invent.kde.org/documentation/develop-kde-org/-/issues/158

## Design goals
- Be brief and only include actionable information; no filler text or
  endless philosophizing.
- Be more prescriptive than descriptive, because descriptiveness is
  impossible when the things being described are themselves currently
  inconsistent. Instead, make recommendations based on KDE's philosophy
  and most successful designs.
- Focus on QtQuick for specific code/component recommendations.
- ...But rather than offer details about how to implement every component
  under the sun, let their API documentation, default settings, and
  existing implementations do the explaining.
- Primary audience is app developers, not Plasma developers or VDG icon
  designers. Plasma's core UI is different from that of an app, and a more
  focused HIG for it can be done later if needed.
- Don't focus on visual styling, as the default style itself should be the
  source of truth for this, and our styling is also in some flux right now.
  Details about styling for custom components can be added later.