[FR] Import multiple folder of markdown

### Description

The current import function only accept a single folder, which is inconvenient if a user has multiple folders.

### Impact

Makes it easier to import larger volume of data locally.

### Additional Context

_No response_

Start by tracing the existing import function for markdown folders and run its current import flow to understand the single-folder behavior. Done means users can select and import multiple folders in one operation, with the imported content from each folder available locally.

Written by the indexing model from the issue text.