[Bug] export as markdown doesn't support linked pages

### Bug Description

I've create a sort of template for writing novel in AppFlowy using the snowflake methods. It's basically a copy of manuskript UI. I want to share it, but exporting the main page in markdown with other linked page doesn't really help, the page links shows as $ in the markdown. If we could export some kind of compressed file, I think we could create shareable templates

### How to Reproduce

Export a page which includes linked pages

### Expected Behavior

can export everything inside a page

### Operating System

macOS

### AppFlowy Version(s)

v0.3.7

### Screenshots

_No response_

### Additional Context

_No response_

Reproduce the markdown export for a page containing linked pages on macOS, using the reported AppFlowy v0.3.7 behavior as a baseline. Trace the export entry point and determine how linked pages are represented; done means exporting a page includes its linked pages in a usable shareable form rather than rendering their links as "$".

Written by the indexing model from the issue text.