Add Synthetic Monitoring as category to home page for MVC

Why is this change being made?

This MR adds Synthetic Monitoring as a category to the homepage for the Monitor stage.

Currently Digital Experience Management is not being prioritized However the tools being used in this category is very similar to what the Quality Department is using to test GitLab. The Quality team is going to ship their new load testing feature as an MVC for synthetic monitoring in the next few releases.

Author Checklist

• Correct MR template applied (e.g blog post)
• 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 your manager.
  • 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.

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