feat: add jsonParse flag to mongooseAdapter that preserves existing, untracked MongoDB data types (#7338)

## Description

Preserves external data structures stored in MongoDB by avoiding the use
of `JSON.parse(JSON.stringify(mongooseDoc))`.

- [x] I have read and understand the
[CONTRIBUTING.md](https://github.com/payloadcms/payload/blob/main/CONTRIBUTING.md)
document in this repository.

## Type of change

- [x] New feature (non-breaking change which adds functionality)

## Checklist:

- [x] I have added tests that prove my fix is effective or that my
feature works
- [x] Existing test suite passes locally with my changes

docs/database/mongodb.mdx

@@ -34,6 +34,7 @@ export default buildConfig({
 |----------------------|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
 | `autoPluralization`  | Tell Mongoose to auto-pluralize any collection names if it encounters any singular words used as collection `slug`s.                                                                                                                                                                          |
 | `schemaOptions`      | Customize schema options for all Mongoose schemas created internally.   |
+| `jsonParse`          | Set to false to disable the automatic JSON stringify/parse of data queried by MongoDB. For example, if you have data not tracked by Payload such as `Date` fields and similar, you can use this option to ensure that existing `Date` properties remain as `Date` and not strings. |
 | `collections`        | Options on a collection-by-collection basis. [More](#collections-options)  |
 | `globals`            | Options for the Globals collection created by Payload. [More](#globals-options) |
 | `connectOptions`     | Customize MongoDB connection options. Payload will connect to your MongoDB database using default options which you can override and extend to include all the [options](https://mongoosejs.com/docs/connections.html#options) available to mongoose.                                         |
@@ -74,4 +75,28 @@ const db = mongooseAdapter({
 
 ### Global Options
 
-Payload automatically creates a single `globals` collection that correspond with any Payload globals that you define. When you initialize the `mongooseAdapter`, you can specify settings here for your globals in a similar manner to how you can for collections above. Right now, the only property available is `schemaOptions` but more may be added in the future.
+Payload automatically creates a single `globals` collection that correspond with any Payload globals that you define. When you initialize the `mongooseAdapter`, you can specify settings here for your globals in a similar manner to how you can for collections above. Right now, the only property available is `schemaOptions` but more may be added in the future.
+
+### Preserving externally managed data
+
+You can use Payload in conjunction with an existing MongoDB database, where you might have some fields "tracked" in Payload via corresponding field configs, and other fields completely unknown to Payload.
+
+If you have external field data in existing MongoDB collections which you'd like to use in combination with Payload, and you don't want to lose those external fields, you can configure Payload to "preserve" that data while it makes updates to your existing documents.
+
+To do this, the first step is to configure Mongoose's `strict` property, which tells Mongoose to write all data that it receives (and not disregard any data that it does not know about).
+
+The second step is to disable Payload's automatic JSON parsing of documents it receives from MongoDB.
+
+Here's an example for how to configure your Mongoose adapter to preserve external collection fields that are not tracked by Payload:
+
+```ts
+mongooseAdapter({
+  url: process.env.DATABASE_URI,
+  // Disable the JSON parsing that Payload performs
+  jsonParse: false,
+  // Disable strict mode for Mongoose
+  schemaOptions: {
+    strict: false,
+  },
+})
+```