[FR] export function

### Feature Description

export function in the web version.
I want to download Markdown format, HTML, or even PDF through export.

### Use Cases

can also use the export function in the web version
Can be used without being dependent on the app
It can be used for educational materials or blogs

### User Impact

First, it is very useful when transferring data using a general MD.
Second, if you create a PDF using MD, it can be used to distribute educational materials or convey information.

### Potential Implementation Ideas

_No response_

### Additional Context

_No response_