Basic requirements:

• Be able to maintain the documentation for software project as part of the source code (in version control system). This enables versioning, branching, merging of the documentation.
• The documentation should be easy to write. HTML is too verbose. Markdown is too unstructured. Wikitext is somewhere in the middle.
• An open source C# library for this.

Solution:

• Maintain the documentation in wikitext files under the version control system.
• Provide tools to generate various artifacts from such documentation (one example would be HTML export).
• The exported docs can then be published online or packaged together with the software product.

NOTE: this is not a replacement for source code documentation generators like doxygen, javadocs, sandcastle and similar.

For an example of what Quiki produces, take a look at this: http://maperitive.net/docs/manual/