Split documentation into files by topic and convert to AsciiDoc format.

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+HTML processing and layout
+==========================
+
+The modules in the layout directory process and layout HTML pages.
+
+Overview
+--------
+This is the process to render an HTML document:
+
+First the HTML is parsed to a tree of xmlNodes using the HTML parser in libxml.
+This happens simultaneously with the fetch [html_process_data()].
+
+Any stylesheets which the document depends on are fetched and parsed.
+
+The tree is converted to a 'box tree' by xml_to_box(). The box tree contains a
+node for each block, inline element, table, etc. The aim of this stage is to
+determine the 'display' or 'float' CSS property of each element, and create the
+corresponding node in the box tree. At this stage the style for each element is
+also calculated (from CSS rules and element attributes). The tree is normalised
+so that each node only has children of permitted types (eg. TABLE_CELLs must be
+within TABLE_ROWs) by adding missing boxes.
+
+The box tree is passed to the layout engine [layout_document()], which finds the
+space required by each element and assigns coordinates to the boxes, based on
+the style of each element and the available width. This includes formatting
+inline elements into lines, laying out tables, and positioning floats. The
+layout engine can be invoked again on a already laid out box tree to reformat it
+to a new width. Coordinates in the box tree are relative to the position of the
+parent node.
+
+The box tree can then be rendered using each node's coordinates.