Fix broken links in package direction and group

Why is this change being made?

As part of gitlab-com/marketing/inbound-marketing&251 (closed), this MR fixes broken, duplicated links and adds title pages to help drive SEO for the Package stage and Group.

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
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  • 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)