restore JSON syntax highlighting (#1505)

At some point between the v0.24 site and the v0.25 site we lost syntax
highlighting on JSON code blocks. (I assume the set of default languages
changed at some point.) Update the docusaurus config file to restore
JSON syntax highlighting.

Author: kenjenkins

docusaurus.config.js

@@ -222,6 +222,7 @@ const config = {
         'actionscript',
         'log',
         'ini',
+        'json',
         'nginx',
         'rego',
         'shell-session',