Adding a page vm.html as a stable target for our virtual machine image

This new page redirects to wherever the VM is actually located.

Note: I originally tried to create a generic page `_layouts/redirect.html`
that would redirect to `{{page.redirect}}`, and then set the key `redirect`
in the page `vm.html` like this:

    redirect: {{site.vm_url}}

This doesn't work: Jekyll doesn't appear to expand variables in key/value
settings in the header of one page to pass into another.