[FEATURE] Import document notes

After reading an article, I often create a summary of the key takeaways in the 'document note' text field of Readwise. This is the most important piece of content to synchronise with RemNote.

No files, tests, or entry points are named. Start by locating the existing Readwise-to-RemNote import or synchronization path and how document metadata is handled; done means the document note text is included in synchronization. Confirm the scope with maintainers before coding, since the issue has had no recent activity.

Written by the indexing model from the issue text.