Add placeholders for AMAU/GMAU package

Why is this change being made?

On a call with @kathleentam and @jeromezng we determined that we should add GMAU/SMAU/AMAU metric placeholders in the handbook while we work on the implementation of instrumentation and measuring them via the package dashboard.

Changes

• Added GMAU/Paid GMAU/SMAU/Paid SMAU as separate entries to match the project planning documentation.
• Added two new AMAUs to break out packages published vs. installed. Which is important because one it shows the difference between creators vs. consumers.

Author Checklist

• Provided a concise title for the MR
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