Added skeletons for docs on customizing pandoc and using pandoc API.

See #3289 and  #3288.

doc/customizing-pandoc.md

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+# Customizing pandoc
+
+## Templates
+
+## Reference docx/odt
+
+## Custom lua writers
+
+## Custom syntax highlighting
+
+syntax definitions, styles
+
+## Filters
+
+including documentation of the JSON serialization format and
+AST definition
+
+

doc/using-the-pandoc-api.md

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+# Using the pandoc API
+
+## Concepts
+
+## Basic usage
+
+## The Pandoc structure
+
+## Reader options
+
+## Writer options
+
+## The PandocMonad class
+
+custom PandocMonad instances
+
+## Builder
+
+example: report from CSV data
+
+## Generic transformations
+
+Walk and syb for AST transformations
+
+## Filters
+
+writing filters in Haskell
+