tpl/tplimpl: Optionally exclude content from sitemap

Define global inclusion/exclusion in site configuration, and override
via front matter. For example, to exclude a page from the sitemap:

    [sitemap]
    disable = true # default is false

Closes #653
Closes #12282

Co-authored-by: kolappannathan <kolappannathan@users.noreply.github.com>
Co-authored-by: felicianotech <FelicianoTech@gmail.com>

docs/data/docs.yaml

@@ -1649,6 +1649,7 @@ config:
       disableInlineCSS: false
   sitemap:
     changeFreq: ""
+    disable: false
     filename: sitemap.xml
     priority: -1
   social: null
@@ -2797,8 +2798,8 @@ tpl:
             {{ $m.Set "Hugo" "Rocks!" }}
             {{ $m.Values | debug.Dump | safeHTML }}
           - |-
-            map[string]interface {}{
-              "Hugo": "Rocks!",
+            {
+              "Hugo": "Rocks!"
             }
       TestDeprecationErr:
         Aliases: null