The following should be a maxim (aka fundamental rule) in every software project:
If it’s not under version control, it does not exist.
One way which helped me several times is the following:
- When you release your software (in continuous deployment scenarios, you might stick to major releases…), create a pdf file from your documentation.
- Put this pdf under version control, in the same branch, tag/label or release info as your code-to-be-released.
Don’t accept a lack of version control!
Really (really!) make sure you find a (mostly automated) way to get your documentation under version control.