Documented next/previous_week. Refs #10890.

    * ``week``: A ``datetime.date`` object representing the first day of

      the given week.

    * ``next_week``: A ``datetime.date`` object representing the first day

      of the next week. If the next week is in the future, this will be

      ``None``.

    * ``previous_week``: A ``datetime.date`` object representing the first

      day of the previous week. Unlike ``next_week``, this will never be

      ``None``.

    **Notes**

    * Uses a default ``template_name_suffix`` of ``_archive_week``.