I fantasize about an environment where a GUI's internals and interfaces are annotated sufficiently to discretely call from unit and integration tests, and version-controlled instances of those are linked to non-technical task management, user documentation, support tickets, support knowledge bases, patch configuration management, and so on. In this fantasy, if code changes a GUI element enough to alter its appearance or fail a unit test of the GUI, it automatically triggers a re-imaging of snapshots for it in all downstream linked documentation references and issues PR's for human intervention reviews of the new images (with adjustable cropping for finalized approvals).
That's my holy grail of single-source documentation.
Where do your business critical spreadsheets go?
The exception is engineering documentation: API docs should absolutely be version-controlled in the same place as the code they are describing.
I built an internal search engine for a past employer that covered 11 different sources of documentation - Confluence, Google Docs, GitHub Markdown, Salesforce Support and more - and it really helped solve some of the discoverability problems we were having.
For product-level user-facing documentation written by non-engineers I think it's entirely appropriate to use a different tool.