Doc: add tasklist and checklist QDoc macros

Adds two reusable QDoc macro sets for interactive documentation widgets,
along with their CSS and JavaScript resources.

\tasklist / \taskstep / \tasksection / \endtasklist
  Renders a multi-step guide with a two-column layout: a sidebar of
  radio-button-driven step labels on the left and a content panel on
  the right. Only the active panel is visible. Switching steps is pure
  CSS using :checked ~ sibling selectors, following the same mechanism
  as the built-in \tab macro. Previous/Next navigation buttons and a
  progress bar are injected automatically by tasklist.js based on the
  order of \taskstep declarations — no per-step arguments required.

\checklist / \checklistsection / \endchecklist
  Renders a grouped checklist with one card per section. Items are
  written using standard \list / \li QDoc commands. checklist.js
  injects a checkbox <input> into each <li> at runtime, so \c{},
  \l{}, and other QDoc commands work freely inside list items.
  Checking an item strikes through its label via the CSS
  :checked ~ span rule. A per-section progress bar and percentage
  counter are updated by checklist.js as items are checked.

Both widgets adapt to light and dark themes via CSS custom properties
and prefers-color-scheme, using Qt documentation style conventions:
Titillium Web font, the standard body text and heading colors from
online.css, and '#41cd52' as the Qt green accent color.

Change-Id: I6fa374d69760f9b7f6da80862ed582a87d5f284d
Reviewed-by: Topi Reinio <topi.reinio@qt.io>