fix: support escaped characters in markdown backend (#2304)

fix: improve markdown backend to support input documents with escaped characters

Signed-off-by: Lucas Morin <lucas.morin222@gmail.com>

tests/data/groundtruth/docling_v2/escaped_characters.md.md

@@ -0,0 +1,41 @@
+# Headers:
+
+## & < > " '
+
+Text: 00:16.000 ----> 00:18.000 & < > " '
+
+# Lists
+
+1. & < > " '
+
+- & < > " '
+
+# Inline code
+
+```
+& < > " '
+```
+
+# Code block
+
+```
+& < > " '
+```
+
+# Table
+
+| Key          | Example   |
+|--------------|-----------|
+| Ampersand    | &         |
+| Less-than    | <         |
+| Greater-than | >         |
+| Quotes       | "         |
+| Apostrophes  | '         |
+
+# Raw HTML
+
+&amp; &lt; &gt; " '/div&gt;
+
+## Link
+
+[&amp; &lt; &gt; " '](https://en.wikipedia.org/wiki/Albert_Einstein)