Provide make target to update lua module docs (#3946)

The pandoc module documentation in doc/lua-filters.md was automatically
generated from `data/pandoc.lua`.  A make target is provided which uses
a lua filter to update the documentation.

tools/update-lua-docs.lua

@@ -0,0 +1,32 @@
+local in_module_section = false
+
+function pandoc_module_blocks()
+  local tmp_folder = os.tmpname()
+  os.remove(tmp_folder)
+  os.execute("mkdir -p " .. tmp_folder)
+  os.execute("ldoc -q -l tools -d " .. tmp_folder .. " data/pandoc.lua")
+  local module_file = io.open(tmp_folder .. "/index.html")
+  local module_html = module_file:read("*a")
+  local module_doc = pandoc.read(module_html, "html")
+  return module_doc.blocks
+end
+
+function Header (el)
+  if in_module_section then
+    if el.level == 1 then
+      in_module_section = false
+      return el
+    else
+      return {}
+    end
+  elseif el.identifier == "module-pandoc" then
+    in_module_section = true
+    return pandoc_module_blocks()
+  end
+end
+
+function Block (el)
+  if in_module_section then
+    return {}
+  end
+end