Update Dependency management section in the Azure Functions PowerShell developer guide

Please update the **Dependency management** section at https://learn.microsoft.com/en-us/azure/azure-functions/functions-reference-powershell?tabs=portal#dependency-management.

For dependency management, there are two options:
1) Managed dependencies (via `requirements.psd1` and `host.json`) -- these are managed by the Functions service
2) Include function app dependencies (PowerShell modules) with the function app content. These dependencies can be placed in the `Modules` folder at the function app root. This information is currently under the `Function app-level modules folder` section.