Enable code-folding in pkgdown vignettes

Code-folding isn't directly supported by pkgdown; see:

    https://github.com/r-lib/pkgdown/issues/1433

We can either wrap code blocks in `<details>...</details>`, or enforce
the original output format by specifying `as_is: true` in the YAML
metadata.

vignettes/evidence-for-parameter-values.Rmd

@@ -4,8 +4,13 @@ bibliography: articles/references.bib
 csl: articles/journal-of-theoretical-biology.csl
 link-citations: TRUE
 output:
-  rmarkdown::html_vignette:
+  bookdown::html_document2:
+    base_format: rmarkdown::html_vignette
     code_folding: show
+    toc: true
+    number_sections: false
+pkgdown:
+  as_is: true
 vignette: >
   %\VignetteIndexEntry{Evidence for parameter values}
   %\VignetteEngine{knitr::rmarkdown}

vignettes/rbc-steady-state.Rmd

 ---
 title: "Red blood cell steady-state distribution"
-output: rmarkdown::html_vignette
+output:
+  bookdown::html_document2:
+    base_format: rmarkdown::html_vignette
+    code_folding: show
+    toc: true
+    number_sections: false
+pkgdown:
+  as_is: true
 vignette: >
   %\VignetteIndexEntry{Red blood cell steady-state distribution}
   %\VignetteEngine{knitr::rmarkdown}

vignettes/sensitivity-analyses.Rmd

 ---
 title: "Sensitivity analyses for Pf and Pv"
 output:
-  rmarkdown::html_vignette:
+  bookdown::html_document2:
+    base_format: rmarkdown::html_vignette
     code_folding: show
+    toc: true
+    number_sections: false
+pkgdown:
+  as_is: true
 vignette: >
   %\VignetteIndexEntry{Sensitivity analyses for Pf and Pv}
   %\VignetteEngine{knitr::rmarkdown}

vignettes/spleenrbc.Rmd

 ---
 title: "Introduction to spleenrbc"
 output:
-  rmarkdown::html_vignette:
+  bookdown::html_document2:
+    base_format: rmarkdown::html_vignette
     code_folding: show
+    toc: true
+    number_sections: false
+pkgdown:
+  as_is: true
 vignette: >
   %\VignetteIndexEntry{Introduction to spleenrbc}
   %\VignetteEngine{knitr::rmarkdown}

vignettes/uninfected-rbc-loss.Rmd

 ---
 title: "Uninfected red blood cell loss due to malaria"
 output:
-  rmarkdown::html_vignette:
+  bookdown::html_document2:
+    base_format: rmarkdown::html_vignette
     code_folding: show
+    toc: true
+    number_sections: false
+pkgdown:
+  as_is: true
 vignette: >
   %\VignetteIndexEntry{Uninfected red blood cell loss due to malaria}
   %\VignetteEngine{knitr::rmarkdown}