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.

man/use_civilytics_revealjs.Rd

@@ -0,0 +1,27 @@
+% Generated by roxygen2: do not edit by hand
+% Please edit documentation in R/quarto.R
+\name{use_civilytics_revealjs}
+\alias{use_civilytics_revealjs}
+\title{Install the Civilytics Reveal.js extension}
+\usage{
+use_civilytics_revealjs(path = ".", force = FALSE)
+}
+\arguments{
+\item{path}{Character. Project directory to install into. Default `"."`.}
+
+\item{force}{Logical. Overwrite existing files? Default `FALSE`.}
+}
+\value{
+Invisible `NULL`.
+}
+\description{
+Copies the Civilytics Reveal.js Quarto extension into
+`_extensions/civilytics-reveal/` under `path`, including brand logos
+from the package.  After running this function, add
+`format: civilytics-reveal-revealjs` to your `.qmd` YAML front-matter.
+}
+\examples{
+\dontrun{
+use_civilytics_revealjs()
+}
+}