I personally don't see why it's important to use markup, markdown, word processors.
The main thing is to make it easy to contribute. People don't want to learn more than they need to in order to contribute. Expecting them to is completely stupid. Do the most amount of work with the least amount of effort is always the best way of doing things. As such, people won't want to learn any special formatting language just to write something they can do in a word processor. That's what they already know.
Having people contribute stuff and having us format it into markdown would be more effort for us. Provide them an ODT they can open at home and they can format it themselves. Of course, that doesn't take into account stupidity, but we can make guidelines for that so things we get are sensible enough that it minimizes effort of correcting it.
If we do use some sort of markup, I think MediaWiki would be our best bet. That's what Wikipedia uses, so most people will already be familiar with it. We could even set one up separate from the main wiki just for the manuals. If we keep the database and files completely separate, it shouldn't be too hard to delete it later if we decide to.
The main problem either case is converting it to whatever other format. PDFs shouldn't be a problem. But then there will be formatting them for the TDE Help system, and HTML.
Since I already know enough HTML to do normal documentation, and I already have an IDE that I'm well adjusted to, I wouldn't mind helping on that respect. I can also help writing the manuals. If I have to learn something new to do that when I can easily do it with what I already know, count me out.