docs(website): Migrate to updateMany, updateOne, upsertOne

docs/website/docs/migrate-from-kmongo/update.md

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+# Update data
+
+Much like [find variants](search.md), update operations use a DSL instead of passing raw BSON values.
+
+## Updating multiple documents
+
+Like with KMongo, the function to update multiple documents is `updateMany`.
+
+```kotlin title="With KMongo"
+collection.updateMany(
+	filter = and(
+		User::name.exists(),
+		User::age gt 18
+	),
+	set(
+		User::isLegal setTo true
+	)
+)
+```
+
+```kotlin title="With KtMongo"
+collection.updateMany(
+	filter = {
+		User::name.exists()
+		User::age gt 18
+	},
+	update = {
+		User::isLegal set true
+	}
+)
+```
+
+To learn more about filtering, visit [the find documentation](search.md).
+
+Unlike in KMongo, there is no need to combine multiple operators yourself, the library will do it for you. The order of operators is not relevant.
+
+```kotlin title="With KMongo"
+collection.updateMany(
+	filter = …,
+	set(
+		User::name setTo "foo",
+		User::isLegal setTo true,
+	),
+	inc(
+		User::age setTo 1
+	)
+)
+```
+
+```kotlin title="With KtMongo"
+collection.updateMany(
+	filter = { … },
+	update = {
+		User::name set "foo"
+		User::isLegal set true
+		User::age inc 1
+	}
+)
+```
+
+## Updating a single document
+
+Like in KMongo, KtMongo provides a dedicated function to edit a single element, `updateOne`. 
+
+## Inserting a document if it doesn't exist
+
+KtMongo provides an overload to perform upserts:
+
+```kotlin title="With KMongo"
+collection.updateOne(
+	filter = …,
+	setOnInsert(
+		User::creationDate setTo Instant.now(),
+	),
+	UpdateOptions().upsert(true)
+)
+```
+
+```kotlin title="With KtMongo"
+collection.upsertOne(
+	filter = { … },
+	update = {
+		User::creationDate setOnInsert Instant.now()
+	}
+)
+```
+
+!!! tip
+    It is also possible to perform an upsert by setting `UpdateOptions().upsert(true)`, as well as setting any other update options, with `updateOne` and `updateMany`, just like in KMongo. `upsertOne` is just syntactic sugar, since we find that we use it very often.
+
+    Note that options are often the first parameter instead of the last (because the last is the lambda block). Other than that, they behave identically.