start mentioning easyform alongside PloneFormGen

[tkimnguyen]

This is related to #711 but since easyform is not as fully featured as PFG it can't really (yet) be considered a replacement.

However, Plone 5.2 on Python 3 will not run PFG, which is Archetypes-based, making easyform the only viable TTW form tool.

With that in mind, we need to start mentioning easyform in the docs where PFG is mentioned or documented, e.g. https://docs.plone.org/working-with-content/managing-content/ploneformgen/index.html