Change the file type of introduction.md

[troyraen]

There is a new documentation page at /notebooks/introduction.md that is automatically open when starting a server. I think that is a good thing, except that the markdown format now triggers it to run as a notebook. Running as a notebook means that the user has to do 5 point-and-clicks to fully close it (2 clicks) and shut down the kernel (3 clicks). Can the file type be changed to something like .pdf or .html so that it only takes one click to close it?

[troyraen]

I'm not sure which repo controls this file, so let me know if this isn't the right place for the issue.

[bsipocz]

I think it would be nice if the file would be in the documentation repo, so everything is in one place.