Add invitations index for home view (#186)

# Add invitations index for home view

## ♻️ Current situation & Problem
Querying upcoming appointments doesn't work on staging, because of
missing index.


## ⚙️ Release Notes 
* Add invitations index for home view


## ✅ Testing
I came with this index after testing on `dev` environment.


### Code of Conduct & Contributing Guidelines 

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of
Conduct](https://github.com/StanfordBDHG/.github/blob/main/CODE_OF_CONDUCT.md)
and [Contributing
Guidelines](https://github.com/StanfordBDHG/.github/blob/main/CONTRIBUTING.md):
- [x] I agree to follow the [Code of
Conduct](https://github.com/StanfordBDHG/.github/blob/main/CODE_OF_CONDUCT.md)
and [Contributing
Guidelines](https://github.com/StanfordBDHG/.github/blob/main/CONTRIBUTING.md).

firestore.indexes.json

@@ -14,6 +14,24 @@
         }
       ]
     },
+    {
+      "collectionGroup": "invitations",
+      "queryScope": "COLLECTION",
+      "fields": [
+        {
+          "fieldPath": "user.clinician",
+          "order": "ASCENDING"
+        },
+        {
+          "fieldPath": "user.organization",
+          "order": "ASCENDING"
+        },
+        {
+          "fieldPath": "user.type",
+          "order": "ASCENDING"
+        }
+      ]
+    },
     {
       "collectionGroup": "users",
       "queryScope": "COLLECTION",