Add /page/documentation

Like Lichess: lichess.org/page/documentation

For example, if I want to read about the meaning of KIF headers, the only page that contains a link to lishogi.org/explanation/kif is this blog post: lishogi.org/blog/YTpk6RAAACUAI5dL/kif-imports-and-exports

With lishogi.org/page/documentation , all links would be easily accessible for users (and the link to Documentation shold be accessible from the menu).

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