The constributing page is not a plain markdown any more.
It is part of the generated webhelp, so link to this. Some of the links
were still to the symbolic link at the root of the repository.

The docs are availabe as webhelp, which is a nicer viewer than
the plain markdown at GitHub. So a little search & replace:
- "(./docs" --> "https://mermaid.js.org"
  vim command ":%s/(.\/docs/(https:\/\/mermaid.js.org/g"
- ".md)" --> ".html)"
  vim command ":%s/.md)/.html)/g"

Replace each link to mermaid-js.github.io with corresponding link
at the webhelp (/ website), using the redirects to get these links.
(The redirects are to the http:// sites. I replace it with the https://
links of course.)