Remove deprecated documentation

[ggrossetie]

As far as I understand, the documentation is now at https://github.com/mermaid-js/mermaid/tree/develop/docs

To avoid confusion, I think we should remove all the content in this repository, except the index.html to redirect from mermaidjs.github.io to mermaid-js.github.io.

We should also make it clear that the only purpose of this repository is to create a redirection from mermaidjs.github.io to mermaid-js.github.io.