Add formatter for consistent markdown formatting

[JammingBen]

It would be nice to have a formatter so all markdown files are formatted the same way. It seems that different IDEs are using different formatting rules, resulting in an inconsistent format across all markdown files. And it's pretty annoying for all folks that have "format on save" enabled in their IDE, since it might re-format whole files despite you simply wanting to change one sentence.

I suggest using prettier with the @opencloud-eu/prettier-config config since it's already well established and easy to integrate.