docs: add deprecation warnings for mongoose-autopopulate plugin #15745

[Ayush123-e]

Summary

Addresses issue #15745 by adding cautionary notices for mongoose-autopopulate across documentation. Informs users it's an external plugin and recommends verifying maintenance status and version compatibility before production use.

Examples

Added warning in populate.md:

> [!WARNING]
> mongoose-autopopulate is an external plugin not maintained by the Mongoose team. Verify it is actively maintained and compatible with your Mongoose version.

Added middleware alternative:
MySchema.pre('find', function() {
  this.populate('user');
});

[Ayush123-e]

@vkarpov15 Please review.

[vkarpov15]

This is spam, please stop.