wip: Include markdown content, via pandoc
Use pandoc to convert freestanding markdown files to docbook for inclusion in the generated docs, and use bits and pieces of gtk-doc code to continue expanding typical gtk-doc abbreviations. The new tool for markdown -> docbook is a python script called gtk-markdown-to-docbook. The markdown dialect is specified via a list of pandoc extension in gtk-markdown-to-docbook. It includes header annocations, definition lists and tables, among other things. This commit converts the 3 overview chapters (drawing, input handling and actions) and the migration guide to markdown syntax. Other files that are still listed in content_files can be converted later. This commit adds a pandoc dependency.
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