docs: add Recommended Integrations page

Adds docs/integrations.html, a new reference page covering 16
best-in-class third-party libraries that pair well with AdminLTE.

Every library is MIT-licensed, actively maintained, and jQuery-free.
For each one the page lists a CDN install snippet and the minimum JS
needed to wire it up:

- Form widgets: Flatpickr (date/time), Tom Select (multi-select / tag),
  noUiSlider (range), Pickr (color), IMask (input mask)
- File handling: Dropzone, FilePond
- Editors: Quill (WYSIWYG), EasyMDE (markdown), Toast UI Editor
- Charts: ApexCharts, Chart.js
- Data tables: Tabulator (already used in tables/data.html)
- Calendar: FullCalendar (already used in pages/calendar.html)
- Drag-and-drop: SortableJS (already used in pages/kanban.html)
- Lightbox: GLightbox
- Icon libraries: Bootstrap Icons, Lucide, Tabler Icons, Material
  Symbols, FontAwesome Free, Phosphor (comparison table)

AdminLTE deliberately bundles none of these -- the page exists so users
can pick what they need without expanding the framework's footprint.

Resolves the recurring "where is the markdown editor / date picker /
file uploader" question (most recently #6022).