Allow building with upstream www/gohugo.

[fernape]

Summary

When we build our documentation and website we use a hacked version of
www/gohugo that allows for arbitrary asciidoc extensions. This was added as a
patch in r562416.

The suggested pull request[1] this patch was based on was not accepted "as is"
and only allows the execution of extensions in Ruby's $LOAD_PATH.

With these changes, we add our <docroot>shared/lib directory to Ruby's library
directory so we can load our custom extensions. This also allows building the
docs and website outsite FreeBSD using upstream and unmodified tools.

Tested with the upcoming vanilla gohugo-0.82 provided by gohugo's maintainer.

[1] https://github.com/gohugoio/hugo/pull/8131

Test Plan

• Apply patch
• cd <docroot> && make
• Docs and website build fine and the produced files look good
• Tested with gohugo-0.82 here

[carlavilla]

Thanks for this!