It's not in the Standard skin because not everyone wants, or needs, a smartphone formatted version, so a link is not always needed.
Figure out where you want the link located in the Standard skin index template index.html.tmpl, then place something like this in that location (NOT TESTED):
<a href="smartphone">Smartphone formatted</a>
Then, make sure you enable the smartphone skin! In weewx.conf:
[[SmartphoneReport]]
# The SmartphoneReport uses the 'Smartphone' skin, and the images and
# files are placed in a dedicated subdirectory.
skin = Smartphone
enable = true
HTML_ROOT = public_html/smartphone