Footer cleanups & documentation update

As part of #3133569: Migrate Drupal 8 & 9 documentation to evergreen format (instead of major version specific), I’m doing a spring clean of the footer:

Pick 4-5 links from the sidebar of https://www.drupal.org/documentation. Tentatively:

• Remove Drupal 7 docs
• Update Drupal 8 docs
• Add a link to the Drupal User Guide

Minor cleanups:

• Social media directory → Social media - less words are nice.
• Add Sign up for Drupal news
• Security announcements → Security advisories - the title of the page it links to.
• Collapse any redirects, like https://drupal.org/drupal-services → https://www.drupal.org/drupal-services