Rework docs to use polyglot notebooks

Since fsdocs now supports generation of documentation with notebooks as input, a rework of the docs that also offers side-by-side examples for F# and C# is overdue.

I think this should be done gradually with a mechanism that allows publishing the new and old docs together as long as not all pages are ported.

This also would be a huge step forward for #376 , at least in a way that we could provide the docs in a format that is needed for the official pipeline.

まず、ノートブックベースのドキュメントに対する fsdocs のサポートと、issue #376 の要件を確認します。段階的な移行中に新旧のドキュメントを一緒に公開する方法を明らかにし、そのうえで F# と C# の例を横並びで示す範囲と、公式のパイプライン形式を定義します。