Update & Delete Metrics Pages (#833)

* add link to metric inventory page

* Delete src/cookbooks/active_dau.md

* Delete src/cookbooks/retention.md

* Delete src/cookbooks/dau.md

* Update SUMMARY.md to remove deleted pages

* Update metrics.md w link to metric-hub

* Update terminology.md to clarify DAU definition

src/SUMMARY.md

@@ -47,9 +47,6 @@
       - [Seeing Your Own Pings](cookbooks/view_pings_cep.md)
       - [See Real-time search metrics](cookbooks/real_time_search.md)
     - [Metrics](cookbooks/metrics.md)
-      - [Daily Active Users (DAU and MAU)](cookbooks/dau.md)
-      - [Active DAU (aDAU)](cookbooks/active_dau.md)
-      - [Retention](cookbooks/retention.md)
   - [Operational](cookbooks/operational/index.md)
     - [Creating a Prototype Data Project on Google Cloud Platform](cookbooks/gcp-projects.md)
     - [Creating Static Dashboards with Protosaur](cookbooks/operational/protosaur.md)

src/concepts/terminology.md

@@ -59,7 +59,7 @@ A set of data, which includes ping data, derived datasets, etc.; sometimes it is
 
 ## DAU
 
-Daily Active Users - The number of unique [client ids](#client-id) that we receive a [ping](#ping) from each day.
+Daily Active Users - The number of unique [client ids](#client-id) that are active each day.
 
 For more details, see the [DAU Metric](https://mozilla-hub.atlassian.net/wiki/spaces/DATA/pages/314704478/Daily+Active+Users+DAU+Metric) page on Confluence.
 

src/cookbooks/metrics.md

@@ -1 +1,5 @@
 # Metrics
+
+The most accurate and up-to-date metric information can be found on the [Metrics Inventory](https://mozilla-hub.atlassian.net/wiki/spaces/DATA/pages/620494911/Metrics+Inventory) page in Confluence (Mozilla LDAP required).
+
+The official source code definitions for the metrics can be found in [metric-hub](https://github.com/mozilla/metric-hub/tree/main/definitions) in GitHub.