Move api documentation to a separate folder.

docs/source/api/comment.txt

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+.. include:: ../../../plone/app/discussion/comment.txt

docs/source/api/conversation.txt

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+.. include:: ../../../plone/app/discussion/conversation.txt

docs/source/api/index.txt

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+===
+API
+===
+
+The conversation and replies adapters.
+
+The conversation is responsible for storing all comments. It provides a
+dict-like API for accessing comments, where keys are integers and values
+are IComment objects. It also provides features for finding comments quickly.
+
+The IReplies adapter provides an API for finding and manipulating the comments
+directly in reply to a particular comment (implemented by the CommentReplies
+adpater) or at the top level of the conversation (implemented by the
+ConversationReplies adapter).
+
+
+.. toctree::
+   :maxdepth: 1
+
+   conversation.txt
+   comment.txt