docs: add Getting Started, Customization, RTL, and Migration pages

Closes major gaps in the v4 documentation surfaced during a doc audit
against the page catalog and the TypeScript source:

- **Getting Started** (docs/getting-started.html) -- a 10-minute path
  from zero to a working dashboard using CDN tags. Includes the
  minimum-viable layout markup, an explanation of the app-wrapper
  region classes, a card-widget example, and a "where to go next" map
  pointing at the rest of the docs.

- **Customization & Theming** (docs/customization.html) -- the SCSS
  variables, CSS custom properties, and import-order rules needed to
  retheme AdminLTE. Covers brand colours, sidebar width/breakpoint,
  custom branding, dark-mode-specific overrides, and the
  light-page-with-dark-sidebar pattern used in the demos.

- **RTL Support** (docs/rtl.html) -- documents the existing rtlcss-
  generated stylesheets (which had zero coverage in the docs despite
  shipping in every release). Includes how to enable RTL per-page or
  per-element, what flips automatically, what doesn't, and the
  /*rtl:ignore*/ control directive.

- **Migration from v3** (docs/migration.html) -- the class-rename and
  data-attribute-rename tables (wrapper -> app-wrapper,
  data-widget -> data-lte-toggle, .dark-mode -> data-bs-theme), the
  Bootstrap 4 -> 5 utility renames, jQuery removal, and a suggested
  migration order. Important for the existing v3 user base.

All four pages are wired into the DOCUMENTATIONS section of the
sidenav.