How do I move existing documentation in?

We have about 200 pages of Markdown in a repository, with images and a folder structure that means something.

What survives an import — the tree, the links between pages, the images?

Short answer: import a zip of Markdown or HTML from the documentation header. The folder structure becomes the page tree, relative links between pages are rewritten to their new addresses, and images are uploaded as assets. Front matter titles are used when present.

If your docs live in Git and you want them to keep living there, connect Git Sync instead and treat the repository as the source of truth.