Update Plone how-to

[danielledeleo]

Issue #80 revealed that the Plone docs have changed, and there appears to have been a major release since we wrote our how-to. We should verify that our Plone how-to still works as written and update these broken links.

# from 'make check'

howto/plone.rst:20: [https://docs.plone.org/manage/installing/installation.html] (404 Client Error: Not Found for url: https://6.docs.plone.org/manage/installing/installation.html)
howto/plone.rst:1: [https://docs.plone.org/manage/installing/requirements.html] (404 Client Error: Not Found for url: https://6.docs.plone.org/manage/installing/requirements.html)