Update dependency proxy direction

Why is this change being made?

During the Ops group conversation today it was mentioned that the direction page for the Dependency Proxy didn't clearly outline where we stand today and where we'd like to be in the future. This MR addresses that by:

• Adding in key terms for clarification
• Adding in a comparison of Artifactory to Gitlab
• Identifying next steps and linking to planned epics.

A quick analogy I’ve been using is:

• A hosted registry is like your home kitchen. You can make whatever you want and you can control every aspect of your meal.
• A remote registry is like ordering Postmates. You can get whatever you want but you still have to specify the restaurant and meal.
• A virtual registry is like the Star Trek Replicator. You ask it for something and ‘magically’ it appears.

Once you have the Star Trek Replicator why would you ever cook or order from Postmates again? Although using a virtual registry is not required, it will be the preferred, easy method for customers to publish and install packages. (At least Premium customers)

I added the diagram and a note that using the proxy is optional.

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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