Issue #3515786 by a.dmitriiev: Make MD conversion service pluggable

Closes #3515786

config/install/markdownify.settings.yml

 supported_entity_types:
   - node
   - taxonomy_term
+default_converter: league
+converters:
+  league:
+    header_style: atx
+    suppress_errors: true
+    strip_tags: true
+    strip_placeholder_links: false
+    bold_style: '**'
+    italic_style: '*'
+    remove_nodes: ''
+    hard_break: false
+    list_item_style: '-'
+    preserve_comments: false
+    use_autolinks: true
+    table_pipe_escape: '\|'
+    table_caption_side: 'top'