feat(html): Support formatting tags in HTML texts (#2111)

* add parsing for formatting tags in HTML backend

Signed-off-by: Roman Kayan BAZG <roman.kayan@bazg.admin.ch>

fix latest tests + wiki_duck result files.

Signed-off-by: Roman Kayan BAZG <roman.kayan@bazg.admin.ch>

* convert _collect_parent_format_tags to staticmethod

Signed-off-by: Roman Kayan BAZG <roman.kayan@bazg.admin.ch>

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Signed-off-by: Roman Kayan BAZG <roman.kayan@bazg.admin.ch>

tests/data/groundtruth/docling_v2/formatting.html.md

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+# HTML Text Formatting Examples
+
+This is a **bold (b)** example and right next to it we have a **strong emphasis (strong)** .
+
+Notice that
+
+**strong + bold mixed** looks similar but carries additional semantic meaning.
+
+Here is an *italic (i)* word and an *emphasis (em)* example.
+
+Sometimes we combine them like
+
+*italic + emphasis together* .
+
+Now let's look at text that appears crossed out: ~~strikethrough with s~~ and ~~deleted with del~~ .
+
+You can also mix them:
+
+~~double strikethrough (s + del)~~ .
+
+To highlight insertions or underlines: underlined with u , inserted with ins .
+
+A combination could be:
+
+underline + insertion together .
+
+Subscript and superscript examples:
+
+Water is written as H
+
+2 O using sub.
+
+The mathematical expression x
+
+2 + y 3 uses sup.
+
+They can also be combined: CO
+
+2 * .
+
+Mixing several: This sentence has ***strong + emphasis*** ,
+
+some
+
+**bold + underline** , and a formula like a 2 + b 3 .