Move single posts back to vignettes/singles

pkgdown/_pkgdown.yml

       href: reference/index.html
     themes:
       text: Themes
-      href: articles/themes.html
+      href: articles/singles/themes.html
     fonts:
       text: Fonts
-      href: articles/fonts.html
+      href: articles/singles/fonts.html
     template-vars:
       text: Template Variables
       href: articles/template-variables.html

vignettes/fonts.Rmd → vignettes/singles/fonts.Rmd

 tvv <- xaringanthemer:::template_variables$variable
 ```
 
-```{r child="../man/fragments/_fonts.Rmd"}
+```{r child="../../man/fragments/_fonts.Rmd"}
 ```

vignettes/themes.Rmd → vignettes/singles/themes.Rmd

 knitr::opts_chunk$set(results = "asis", echo=FALSE)
 ```
 
-```{r child="../man/fragments/_themes.Rmd"}
+```{r child="../../man/fragments/_themes.Rmd"}
 ```