markup/goldmark: Enhance footnote extension with backlinkHTML option

This commit introduces a new option, backlinkHTML, to the Goldmark
footnote extension. This allows users to the set custom text for
footnote backlink anchors.

Closes #11434

docs/data/docs.yaml

@@ -1239,6 +1239,7 @@ config:
           superscript:
             enable: false
         footnote:
+          backlinkHTML: '↩︎'
           enable: true
           enableAutoIDPrefix: false
         linkify: true