Short: use *.odt
(LibreOffice Writer) and ReZipDoc (GPLv3) (disclaimer: I maintain it)
Explanation:
Quite some binary formats - among them docx
(MS Word) and odt
(LibreOffice Writer) - are just ZIP files that contain text- and binary-files.
Using git filters, you can re-zip these without compression, which makes them much easier to compress for git, saving a lot of space in git history. This also leaves them in quite diff friendly format, without using an extra diff workflow. Still, most editing software has no problem using these files instead of the compressed ones. The main downside: Each person working on the repo has to install the filter(s)
The ReZipDoc (GPLv3) tool is made for this workflow; it contains a git filter.
The more and bigger binary files (like images) you use within your documents, and the less often they change, the more space you will save each time the text parts of your documents change, compared to not using a filter like this.
I must say though, that technically speaking, and also personally, I would also recommend Markdown over such a solution.
There are already nice GUI editors available for it.