Updating sphinx config (refs #9502)

* adding skins.xml
* using plone-pt theme as override in skins-pt target
Better Plone integration (refs #9502)

* registering documentation folder as resourceDirectory in order to
  avoid future issues as well as the unavailability of folders with
  leading underscores

* updating layout to

    * overwrite the _title_ and _base_ tags from the main_template
    * register theme stylesheets and javascripts
    * register relevant _link_ tags

* updating include paths in the docs .rst files