feat: Implementation of HTML backend with headless browser

[maxmnemonic]

• Implementation of HTML backend that (optionally) uses headless browser (via Playwright) to materialize HTML pages into images, and add provenances with bboxes to all elements in the converted docling document.
• Conversion preserves reading order given by HTML DOM tree
• Added support for HTML "input" fields: checkboxes, radiobuttons, text inputs, etc.
• Added support to Key-Value convention in HTML (i.e. elements with id "key1" and "key1_value1" will be paired as key-values, see test cases as examples)
• Heuristic that glues independent inline HTML elements with single-character text in them into larger text blocks
• Support for inline styling (bold, italic, etc.)

Checklist:

• Documentation has been updated, if necessary.
• Examples have been added, if necessary.
• Tests have been added, if necessary.

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Documentation Updates

1 document(s) were updated by changes in this PR:

What are the detailed pipeline options and processing behaviors for PDF, DOCX, PPTX, and XLSX files in the Python SDK?

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@@ -128,6 +128,68 @@
 
 ---
 
+### HTML
+- **Pipeline/Backend**: `SimplePipeline` + `HTMLDocumentBackend`
+- **Installation Requirements**: HTML rendering (with headless browser support) requires the `htmlrender` extra: `pip install docling[htmlrender]`. This installs Playwright and related dependencies.
+- **Key Options** (`HTMLBackendOptions`):
+    - `render_page` (bool, default: False): Enable headless browser rendering to capture page images and element bounding boxes
+    - `render_page_width` (int, default: 794): Render page width in CSS pixels (A4 @ 96 DPI)
+    - `render_page_height` (int, default: 1123): Render page height in CSS pixels (A4 @ 96 DPI)
+    - `render_page_orientation` (Literal["portrait", "landscape"], default: "portrait"): Page orientation
+    - `render_print_media` (bool, default: True): Use print media emulation when rendering
+    - `render_wait_until` (Literal["load", "domcontentloaded", "networkidle"], default: "networkidle"): Playwright wait condition before extracting DOM
+    - `render_wait_ms` (int, default: 0): Extra delay in milliseconds after load
+    - `render_device_scale` (float, default: 1.0): Device scale factor for rendering
+    - `page_padding` (int, default: 0): Padding in CSS pixels applied to HTML body before rendering
+    - `render_full_page` (bool, default: False): Capture a single full-height page image instead of paginating
+    - `render_dpi` (int, default: 96): DPI used for page images created from rendering
+    - `fetch_images` (bool, default: False): Fetch and embed images from the HTML
+    - `enable_local_fetch` (bool): Enable fetching resources from the local filesystem
+    - `source_uri` (Path or str): Base URI for resolving relative paths in HTML
+- **Processing**:
+    - Reading order is preserved from the HTML DOM tree
+    - Supports HTML form elements: checkboxes, radio buttons, text inputs, and other input fields
+    - Supports key-value pair conventions where HTML elements with matching IDs (e.g., "key1" and "key1_value1") are automatically paired as key-value relationships
+    - When `render_page=True`, uses Playwright headless browser to materialize HTML pages into images
+    - Adds provenances with bounding boxes to all elements in the converted document when rendering is enabled
+    - Can handle local file paths and remote URLs
+    - Heuristic that glues independent inline HTML elements with single-character text into larger text blocks
+    - Support for inline styling (bold, italic, etc.)
+- **Usage Example**:
+
+```python
+from pathlib import Path
+from docling.datamodel.backend_options import HTMLBackendOptions
+from docling.datamodel.base_models import InputFormat
+from docling.document_converter import DocumentConverter, HTMLFormatOption
+
+html_options = HTMLBackendOptions(
+    render_page=True,
+    render_page_width=794,
+    render_page_height=1123,
+    render_device_scale=2.0,
+    render_page_orientation="portrait",
+    render_print_media=True,
+    render_wait_until="networkidle",
+    render_wait_ms=500,
+    render_full_page=True,
+    render_dpi=144,
+    page_padding=16,
+    fetch_images=True,
+)
+
+converter = DocumentConverter(
+    format_options={
+        InputFormat.HTML: HTMLFormatOption(backend_options=html_options)
+    }
+)
+
+result = converter.convert("path/to/file.html")
+doc = result.document
+```
+
+---
+
 ### LaTeX
 - **Pipeline/Backend**: `SimplePipeline` + `LatexDocumentBackend`
 - **Key Options** (`LatexBackendOptions`):

^{How did I do? Any feedback?}  

[ceberam]

@maxmnemonic just looking at the parsing output, it looks much neater, with less noise and with visual elements that get lost in the current implementation. However, looking at the Wiki duck page, I have seen a pattern that seems strange (see below).

[vagenas]

Check comments for details.

[ceberam]

Great!
Just to help code maintenance, some docstrings and code consolidation would be nice afterwards.