Add simplified mixin without struct information

[Chadys]

Since struct informations should only be put in the homepage(s), it seems a bit wasteful to have all those struct fields on every pages. What I propose:

• Create a SeoSimpleMixin (name subject to debate), which does not contain any struct fields and use only seo_meta_panels and seo_menu_panels as seo_panels
• Update the docs to indicate to use this new mixin on non-root pages and to only add {% include "wagtailseo/struct_data.html" %} to the template of pages using SeoMixin (root pages)

I would be willing to make the PR for this.