refactor: add the contentlayer to html-backend (#1040)

* added the contentlayer to html-backend

Signed-off-by: Peter Staar <taa@zurich.ibm.com>

* updated the handle_image function

Signed-off-by: Peter Staar <taa@zurich.ibm.com>

* reformatted code of html backend

Signed-off-by: Peter Staar <taa@zurich.ibm.com>

* test(html): add more info if a test case fails

Signed-off-by: Cesar Berrospi Ramis <75900930+ceberam@users.noreply.github.com>

* refactor(html): put parsed item in body if doc has no header

In case an HTML does not have any header tag, all parsed items are placed in
DoclingDocument's body content layer.
HTML paragraphs ('p' tags) are parsed as text items with paragraph label.
Update test ground truth accoring to the changes above.

Signed-off-by: Cesar Berrospi Ramis <75900930+ceberam@users.noreply.github.com>

* chore: set TextItem label to 'text' instead of 'paragraph'

Signed-off-by: Cesar Berrospi Ramis <75900930+ceberam@users.noreply.github.com>

---------

Signed-off-by: Peter Staar <taa@zurich.ibm.com>
Signed-off-by: Cesar Berrospi Ramis <75900930+ceberam@users.noreply.github.com>
Co-authored-by: Cesar Berrospi Ramis <75900930+ceberam@users.noreply.github.com>

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

@@ -1,62 +1,3 @@
-Main menu
-
-Navigation
-
-- Main page
-- Contents
-- Current events
-- Random article
-- About Wikipedia
-- Contact us
-
-Contribute
-
-- Help
-- Learn to edit
-- Community portal
-- Recent changes
-- Upload file
-
-<!-- image -->
-
-<!-- image -->
-
-<!-- image -->
-
-    - Donate
-        - Create account
-        - Log in
-        - Create account
-        - Log in
-
-Pages for logged out editors
-
-        - Contributions
-        - Talk
-
-## Contents
-
-        - (Top)
-        - 1 Etymology
-        - 2 Taxonomy
-        - 3 Morphology
-        - 4 Distribution and habitat
-        - 5 Behaviour Toggle Behaviour subsection
-            - 5.1 Feeding
-            - 5.2 Breeding
-            - 5.3 Communication
-            - 5.4 Predators
-    - 6 Relationship with humans Toggle Relationship with humans subsection
-        - 6.1 Hunting
-        - 6.2 Domestication
-        - 6.3 Heraldry
-        - 6.4 Cultural references
-- 7 See also
-- 8 Notes Toggle Notes subsection
-    - 8.1 Citations
-    - 8.2 Sources
-- 9 External links
-
 # Duck
 
 - Acèh