Convert links to local files in markdown

[znz]

Original issue is Some links broken in README .
I found two problem.

1. COPYING, CONTRIBUTING.md, etc. are not in https://docs.ruby-lang.org/en/trunk/.
2. links to local files in markdown do not convert to links to converted html files.

I added missing files to .document, but they are still broken links because rdoc does not convert links to local files.

[aycabta]

The README.md uses link notation.

## Contributing

See the file [CONTRIBUTING.md](CONTRIBUTING.md)

I guess that this is just for GitHub and I think that RDoc's behavior is correct. You can use rdoc-ref(example, raw and generated).

[aycabta]

Just to be safe, please use rdoc-ref without extension. For example, [CONTRIBUTING.md](CONTRIBUTING).

[znz]

[COPYING](COPYING) and [win32/README.win32](win32/README.win32) are without extension.
Should they be [COPYING](rdoc-ref:COPYING) and [win32/README.win32](rdoc-ref:win32/README.win32)?
Is there no way to generate valid link on github and rdoc generated html?

[aycabta]

You can use [COPYING](rdoc-ref:COPYING), but [win32/README.win32](rdoc-ref:win32/README.win32) is failed. I guess this is a bug, so I'll survey this issue later.

We can't generate valid links for both of GitHub UI and HTML files by Darkfish. Is it important? I think that GitHub's UI isn't important for Ruby. And, I don't want to optimize RDoc for GitHub.