Fix Sphinx/Doxygen documentation build warnings treated as errors

[Copilot]

sphinx-build -W --keep-going fails due to accumulated warnings across RST files and Doxygen C++ comments. This addresses all warning sources.

Sphinx RST warnings

• Duplicate autosection labels — added autosectionlabel_prefix_document = True to conf.py; common titles like Attributes, Usage, Templating, Utilities collided across files
• shipping_label.rst — added missing blank line between code block and following section heading (docutils "explicit markup ends without blank line")

Doxygen C++ comment warnings

• docraft_color.h / docraft_parser_helpers.h — escaped #RRGGBB/#RRGGBBAA as \#RRGGBB/\#RRGGBBAA; bare #NAME is parsed as a cross-reference link by Doxygen
• docraft_template_engine.h — removed duplicate consecutive /** */ doc blocks on get_template_variable, remove_template_variable, has_template_variable, clear_template_variables; escaped <Image> HTML tags; added missing @param item
• docraft_table.h — added missing @param background to add_title_node, add_htitle_node, add_content_node
• docraft_parser_helpers.h — fixed mismatched @param names: color_attr→color for is_hex_color, color_str→color_attr for get_docraft_color

Doxyfile

• Changed WARN_LOGFILE from ./build/doxygen/warnings.log (directory never created) to ./doxy_files/warnings.log (Doxygen's own OUTPUT_DIRECTORY, created automatically)

Original prompt

Reference: https://github.com/Cadons/Docraft/actions/runs/22842657720/job/66251874012#step:7:1

Correggi i warning e gli errori

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