doc/lua-filters.md: replace metadata example with image centering (#6004)

Metadata defaults can be given via the command line `--metadata-file`.
Adding raw format snippets is a common use case for Lua filters, so it
seems sensible to provide an example.

Thanks to @efx for proposing this filter.

Closes: pandoc/lua-filters#70

doc/lua-filters.md

@@ -264,32 +264,33 @@ return {
 }
 ```
 
-## Default metadata file
+## Center images in LaTeX and HTML output
 
-This filter causes metadata defined in an external file
-(`metadata-file.yaml`) to be used as default values in a
-document's metadata:
+For LaTeX, wrap an image in LaTeX snippets which cause the image
+to be centered horizontally. In HTML, the image element's style
+attribute is used to achieve centering.
 
 ``` lua
--- read metadata file into string
-local metafile = io.open('metadata-file.yaml', 'r')
-local content = metafile:read("*a")
-metafile:close()
--- get metadata
-local default_meta = pandoc.read(content, "markdown").meta
+-- Filter images with this function if the target format is LaTeX.
+if FORMAT:match 'latex' then
+  function Image (elem)
+    -- Surround all images with image-centering raw LaTeX.
+    return {
+      pandoc.RawInline('latex', '\\hfill\\break{\\centering'),
+      elem,
+      pandoc.RawInline('latex', '\\par}')
+    }
+  end
+end
 
-return {
-  {
-    Meta = function(meta)
-      -- use default metadata field if it hasn't been defined yet.
-      for k, v in pairs(default_meta) do
-        if meta[k] == nil then
-          meta[k] = v
-        end
-      end
-      return meta
-    end,
-  }
+-- Filter images with this function if the target format is HTML
+if FORMAT:match 'html' then
+  function Image (elem)
+    -- Use CSS style to center image
+    elem.attributes.style = 'margin:auto; display: block;'
+    return elem
+  end
+end
 ```
 
 ## Setting the date in the metadata