Add Hugo to reference sidebar and warn on missing JSON

Hugo was restored in #1948 but only added to `reference.yml` (index
listing), not to `_quarto.yml` (sidebar). Without the sidebar entry
the page renders without navigation and the reference tables are
empty on deploy previews.

Also add a warning in `reference.lua` when the sibling JSON file
is not found, so silent rendering failures are easier to diagnose.

(cherry picked from commit 55db1b9)

Author: cderv

_quarto.yml

@@ -522,6 +522,8 @@ website:
                       href: docs/reference/formats/markdown/gfm.qmd
                     - text: "CommonMark"
                       href: docs/reference/formats/markdown/commonmark.qmd
+                    - text: "Hugo"
+                      href: docs/reference/formats/markdown/hugo.qmd
                     - text: "Markua"
                       href: docs/reference/formats/markdown/markua.qmd
                 - section: "Wikis"

docs/reference/reference.lua

@@ -39,6 +39,9 @@ function Pandoc(doc)
   local refJsonPath = stem .. ".json"
 
   local refJsonFile = io.open(refJsonPath, "r")
+  if refJsonFile == nil then
+    quarto.log.warning("Reference JSON not found: " .. refJsonPath)
+  end
   if refJsonFile ~= nil then
     io.input(refJsonFile)
     local refJson = io.read("*all")