Enable move a doc inside another doc for pinned docs

[virgile-dev]

Feature Request

Is your feature request related to a problem or unsupported use case? Please describe.
Moving doc in another doc is not easy ...
See #1601

Describe the solution you'd like
We should make it a little easier by making it possible to move a doc inside a pinned docs.
Also we should be able to move a pinned doc in another pin another pinned doc.

Describe alternatives you've considered
Right now users have to update the doc they want to move and the destination doc so that appear higher in their index.

Discovery, Documentation, Adoption, Migration Strategy
If you can, explain how users will be able to use this and possibly write out some documentation (if applicable).
Maybe add a screenshot or design?

Do you want to work on it through a Pull Request?