chore: Add weekly cron job for updating index

[microamp]

Fixes #474

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 96.08%. Comparing base (188f8b6) to head (e337541).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #492   +/-   ##
=======================================
  Coverage   96.08%   96.08%           
=======================================
  Files         153      153           
  Lines        3293     3293           
=======================================
  Hits         3164     3164           
  Misses        129      129           

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[microamp]

The cron job successfully kicked off and completed in staging earlier this week.

NAME                                  READY     STATUS        RESTARTS     AGE
iabwww-cron-weekly-28732350-6mnrp     0/1       Completed     0            2d21h

Container logs:

Updating backend: default
default: Rebuilding index default
default: home.HomePage
default: home.IABHomePage          .
default: blog.BlogPage
default: blog.BlogIndexPage
default: events.EventPage
default: events.EventListingPage
default: forms.FormPage
default: snippets.Charter
default: snippets.WorkingGroup
default: snippets.RFC
default: snippets.Person
default: snippets.Role
default: snippets.Group
default: snippets.CallToAction
default: snippets.MailingListSignup
default: snippets.Topic
default: snippets.Sponsor
default: snippets.GlossaryItem
default: standard.StandardPage
default: standard.StandardIndexPage
default: standard.IABStandardPage  .
default: topics.TopicIndexPage
default: topics.PrimaryTopicPage
default: glossary.GlossaryPage
default: images.IETFImage          .
default: documents.IetfDocument    .
default: iesg_statement.IESGStatementPage
default: iesg_statement.IESGStatementIndexPage
default: announcements.IABAnnouncementPage .
default: announcements.IABAnnouncementIndexPage .
default: wagtaildocs.Document      .
default: wagtailimages.Image
default: wagtailcore.Page          .
default: indexed 70 objects