Make clear that all notebooks need to be converted for mkdocs serve to work

[rasbt]

From the discussion in #915:

In this statement I mean that to check documentation after adding new sections, new contributor should run this command to convert all notebooks to markdown files (which do not exist initially), not just convert new notebook. Otherwise it will not be rendered correctly. Just the note from new contributor :)

I see what you mean. I think in most cases, just modifying the notebook is sufficient. But if someone wants to view it via mkdocs, that would require converting all files to .md. Another thing to make more clear in the contributor guide.