feat(content): add target_elements parameter for selective content extraction

Adds new target_elements parameter to CrawlerRunConfig that allows more flexible content selection than css_selector. This enables focusing markdown generation and data extraction on specific elements while still processing the entire page for links and media.

Key changes:
- Added target_elements list parameter to CrawlerRunConfig
- Modified WebScrapingStrategy and LXMLWebScrapingStrategy to handle target_elements
- Updated documentation with examples and comparison between css_selector and target_elements
- Fixed table extraction in content_scraping_strategy.py

BREAKING CHANGE: Table extraction logic has been modified to better handle thead/tbody structures

docs/md_v2/api/parameters.md

@@ -71,7 +71,8 @@ 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.).                 |
-| **`css_selector`**           | `str` (None)                         | Retains only the part of the page matching this selector.                                       |
+| **`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"]`).                                               |
 | **`excluded_selector`**      | `str` (None)                         | Like `css_selector` but to exclude. E.g. `"#ads, .tracker"`.                                    |
 | **`only_text`**              | `bool` (False)                       | If `True`, tries to extract text-only content.                                                  |