feat: add Quarto themes and templates (revealjs, HTML, PDF, Typst)

Integrate Civilytics Reveal.js slide extension, HTML/PDF/Typst document
themes, and _brand.yml into the package under inst/quarto/. Three new
helper functions (use_civilytics_revealjs, use_civilytics_theme,
use_civilytics_brand) copy assets into a user's Quarto project. Logos
are stored once in inst/img/ and distributed at install time.

R/civilytics-package.R

@@ -19,6 +19,13 @@
 #'   \item [scale_color_civilytics()] / [scale_fill_civilytics()] -- ggplot2 scales
 #' }
 #'
+#' @section Quarto templates:
+#' \itemize{
+#'   \item [use_civilytics_revealjs()] -- install Reveal.js slide extension
+#'   \item [use_civilytics_theme()] -- install HTML/PDF/Typst document theme
+#'   \item [use_civilytics_brand()] -- install brand.yml only (Quarto 1.5+)
+#' }
+#'
 #' @keywords internal
 #' @importFrom stats qbeta
 #' @importFrom stats qnorm