Add docs for diagnostics

docs/docs/administration/CLI_tasks/diagnostics.md

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+# Diagnostics
+
+A few tasks to help with debugging, troubleshooting, and diagnosing problems.
+
+They mostly relate to common postgres queries. 
+
+## Home timeline query plan
+
+This task will print a query plan for the home timeline of a given user.
+
+=== "OTP"
+
+    `./bin/pleroma_ctl diagnostics home_timeline <nickname>`
+
+=== "From Source"
+
+    `mix pleroma.diagnostics home_timeline <nickname>`
+
+## User timeline query plan
+
+This task will print a query plan for the user timeline of a given user,
+from the perspective of another given user.
+
+=== "OTP"
+
+    `./bin/pleroma_ctl diagnostics user_timeline <nickname> <viewing_nickname>`
+
+=== "From Source"
+    
+    `mix pleroma.diagnostics user_timeline <nickname> <viewing_nickname>`