feat(markdown): add content source selection for markdown generation

Adds a new content_source parameter to MarkdownGenerationStrategy that allows
selecting which HTML content to use for markdown generation:
- cleaned_html (default): uses post-processed HTML
- raw_html: uses original webpage HTML
- fit_html: uses preprocessed HTML for schema extraction

Changes include:
- Added content_source parameter to MarkdownGenerationStrategy
- Updated AsyncWebCrawler to handle HTML source selection
- Added examples and tests for the new feature
- Updated documentation with new parameter details

BREAKING CHANGE: Renamed cleaned_html parameter to input_html in generate_markdown()
method signature to better reflect its generalized purpose

docs/md_v2/api/parameters.md

@@ -70,7 +70,7 @@ We group them by category.
 |------------------------------|--------------------------------------|-------------------------------------------------------------------------------------------------|
 | **`word_count_threshold`**   | `int` (default: ~200)                | Skips text blocks below X words. Helps ignore trivial sections.                                 |
 | **`extraction_strategy`**    | `ExtractionStrategy` (default: None) | If set, extracts structured data (CSS-based, LLM-based, etc.).                                  |
-| **`markdown_generator`**     | `MarkdownGenerationStrategy` (None)  | If you want specialized markdown output (citations, filtering, chunking, etc.).                 |
+| **`markdown_generator`**     | `MarkdownGenerationStrategy` (None)  | If you want specialized markdown output (citations, filtering, chunking, etc.). Can be customized with options such as `content_source` parameter to select the HTML input source ('cleaned_html', 'raw_html', or 'fit_html').                 |
 | **`css_selector`**           | `str` (None)                         | Retains only the part of the page matching this selector. Affects the entire extraction process. |
 | **`target_elements`**        | `List[str]` (None)                   | List of CSS selectors for elements to focus on for markdown generation and data extraction, while still processing the entire page for links, media, etc. Provides more flexibility than `css_selector`. |
 | **`excluded_tags`**          | `list` (None)                        | Removes entire tags (e.g. `["script", "style"]`).                                               |