Update links to Jira pages in www repository

Why is this change being made?

I'm most of the way through the work gitlab-org/gitlab#324250 (closed) to polish up our Jira documentation: splitting long pages, reuniting pages in the same directory, and revising to meet tone / style standards. I'll need to make another pass after the capstone work lands -- because the panel integration should get its own clearly-named page in with its buddies, and not be an index page -- I can get at least some of the links repaired now.

Related to gitlab-org/gitlab#324250 (closed)

Author Checklist

• Provided a concise title for the MR
• Added a description to this MR explaining the reasons for the proposed change, per say-why-not-just-what
• Assign this change to the correct DRI
  • If the DRI for the page/s being updated isn’t immediately clear, then assign it to one of the people listed in the "Maintained by" section in on the page being edited.
  • If your manager does not have merge rights, please ask someone to merge it AFTER it has been approved by your manager in #mr-buddies.
  • If the changes relate to any part of the project other than updates to content and/or data files please make sure to ping @gl-static-site-editor in a comment for a review and merge. For example changes to .gitlab-ci.yml, JavaScript/CSS/Ruby code or the layout files. (this requirement has been removed pending identification of a new DRI for the handbook)