Add sales enablement material to gitaly direction

Why is this change being made?

Currently, we're in the process of attempting to enable sales and other customer facing teams to effectively discuss Gitaly Cluster with our users. Adding an enablement section to our direction page allows some easy to find resources for these teams. If we iterate toward having a full enablement section elsewhere, we can maintain a link to that here in its place, but for now this is the MVC approach!

Author Checklist

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• Added a description to this MR explaining the reasons for the proposed change, per say-why-not-just-what
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