docs: add page about app behavior when lookups are unknown

docs/App-behavior-when-lookups-unknown.md

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+# App Behavior When Lookups Are Unknown to CPIC
+
+Lookups are fetched from CPIC and matched with users' diplotypes from the lab.
+Sometimes, genes or diplotypes might be unknown.
+
+This page collects the app's current and desired behavoir in such cases for
+further discussion, as of August 28, 2023.
+
+## App Behavior
+
+| Page | Case | Desired behavior | Current behavior |
+| ----- | ----- | ----------------- | ----------------- |
+| Gene report | Unknown gene | Not shown in gene results |  |
+| Gene report | Unknown diplotype | | |
+| Gene detail | Unknown diplotype | | |
+| Drug search | _TODO_ | | |
+| Drug detail | _TODO_ | | |
+
+_TODO for drug pages: how are guidelines matched? What happens for multiple
+genes, especially if one is missing?_
+
+## Data examples
+
+| Description | Screenshot |
+| ----------- | ------------ |
+| Diplotypes list  |  |
+| Single diplotype |  |
+| Lookups list (matched) |  |
+| Single lookup |  |