docs(plugins): add Overview page, document programmatic API for all 7 plugins

Adds a new Plugins Overview page (javascript/plugins-overview.html)
with an at-a-glance reference table mapping every plugin to its data
API, programmatic methods, and dedicated reference page. Includes
event-name reference, config-via-data-attribute examples, and the
full list of CSS classes the plugins manage on body / app-wrapper /
card / etc.

Each individual plugin page gains a "Programmatic API" section
documenting the real public methods (verified against the TypeScript
source in src/ts/, not invented):

- PushMenu: toggle/expand/collapse/isCollapsed/isExplicitlyOpen/
  isMiniMode/isMobileSize/saveSidebarState/loadSidebarState/
  clearSidebarState/updateStateByResponsiveLogic
- Treeview: open/close/toggle, plus the previously-undocumented
  data-accordion and data-animation-speed attributes
- CardWidget: collapse/expand/toggle/maximize/minimize/
  toggleMaximize/remove, plus the four trigger-selector config keys
- DirectChat: toggle
- FullScreen: toggleFullScreen/inFullScreen/outFullscreen, plus
  notes on the .d-none toggle pattern that #6021 fixed
- Layout: holdTransition(time)

Method signatures, return types, and event fired-on-call mapping
verified against src/ts/{push-menu,treeview,card-widget,direct-chat,
fullscreen,layout}.ts.