chore: adds docs for collection operations

docs/database/overview.mdx

@@ -70,4 +70,105 @@ export default buildConfig({
     }
   }),
 })
-```
+```
+
+## Collection Operations
+
+To configure Collection database operations in your Payload application, your Collection config has methods that can override default database operations for that Collection.
+
+The override methods receive arguments useful for augmenting operations such as Field data, the collection slug, and the req.
+
+Here is an example:
+```ts
+import type { CollectionConfig } from 'payload/types'
+
+export const Collection: CollectionConfig => {
+  return {
+    slug: 'collection-db-operations',
+    db: {
+      // Create a document in a custom db
+      create: async ({ collection, data, req }) => {
+        const doc = await fetch(`https://example.com/api/${collection}/create`, {
+          method: "POST",
+          body: JSON.stringify(data),
+          headers: {
+            'x-app-user': `payload_${req.payload.user}`,
+            'Content-Type': 'application/json'
+          }
+        }).then(response => response.json())
+        
+        return doc
+      },
+
+      // Delete a document in a custom db
+      deleteOne: async ({ collection, data, req }) => {
+        const docs = await fetch(`https://example.com/api/${collection}/delete/${data.id}`, {
+          method: 'DELETE',
+          headers: {
+            'x-app-user': `payload_${req.payload.user}`
+          }
+        }).then(response => response.json())
+        
+        return docs
+      },
+
+      // Delete many documents in a custom db
+      deleteMany: async ({ collection, data, req }) => {
+        const docs = await fetch(`https://example.com/api/${collection}/delete`, {
+          method: 'DELETE'
+          headers: {
+            'x-app-user': `payload_${req.payload.user}`
+          }
+          body: JSON.stringify(data),
+        }).then(response => response.json())
+        
+        return docs
+      },
+
+      // Find documents in a custom db
+      find: async ({ collection, data, req, where, limit }) => {
+        const docs = await fetch(`https://example.com/api/${collection}/find`, {
+          headers: {
+            'x-app-user': `payload_${req.payload.user}`
+          }
+          body: JSON.stringify({data, where, limit}),
+        }).then(response => response.json())
+        
+        return { docs }
+      },
+
+      // Find one document in a custom db
+      findOne: async ({ collection, data, req }) => {
+        const doc = await fetch(`https://example.com/api/${collection}/find/${data.id}`, {
+          headers: {
+            'x-app-user': `payload_${req.payload.user}`
+          }
+        }).then(response => response.json())
+        
+        return doc
+      },
+
+      // Update one document in an custom db
+      updateOne: async ({ collection, data, req }) => {
+        const doc = await fetch(`https://example.com/api/${collection}/update/${data.id}`, {
+          method: 'PUT',
+          body: JSON.stringify(data),
+          headers: {
+            'x-app-user': `payload_${req.payload.user}`,
+            'Content-Type': 'application/json'
+          }
+        }).then(response => response.json())
+        
+        return { ...doc, updated: true }
+      },
+    },
+    fields: [
+      {
+        name: 'name',
+        type: 'text',
+      },
+    ],
+  }
+}
+
+```