Make the linenumbers config.toml changes opt-in

Unfortunately the config.toml is shared by all the examples and so we
try to push example-specific config into the index files. With markdown
configuation that's not possible, so the best we can do is tell the user
they need to comment it in to try it out. The better solution in the
future would be to pull this example out into it's own Hugo site.

exampleSite/config.toml

@@ -19,13 +19,11 @@ name = "Josh Dzielak"
 # defaultMarkdownHandler = "blackfriday"
 [markup.goldmark.renderer]
 unsafe = true
-[markup.highlight]
-# setting the theme for highlighting by Hugo 
-# we need a style that looks good in both light and dark background!
-style = "tango"
-# to allow the use of Highlight.js line numbers and highlights
-# (see https://github.com/hakimel/reveal.js#line-numbers--highlights)
-codeFences = false
+
+# uncomment these lines to run the highlightjs-linenumbers-example
+# [markup.highlight]
+# codeFences = false  # prevent hugo from processing code fences
+# style = "tango"  # hugo highlighting for light + dark background
 
 [outputFormats.Reveal]
 baseName = "index"
@@ -34,8 +32,6 @@ isHTML = true
 
 [params.reveal_hugo]
 history = true
-# setting the theme for highlighting by Highlight.js
-highlight_theme = "github"
 
 # used in content/template-example
 [params.reveal_hugo.templates.grey]