Button links in documentation not rendered correctly

[metc]

Hi,

All buttons in the documentation tutorials have a link defined as follow:

<a href="install.md" class="btn btn-primary btn-lg">Let's Get Started</a>

The rendered html link is not valid (HTTP 404). The link is not converted to the html page link (See https://github.com/coreos/tectonic-docs/blob/master/Documentation/tutorials/sandbox/index.md and https://coreos.com/tectonic/docs/latest/tutorials/sandbox/index.html).

We could replace install.md with install.html. I did not find any other solution to fix this. Please let me know if you have any.

[metc]

Same issue as #81? This bug is only for html tags (used for buttons).