Sarath, I know this is a Scribus forum - and this might sound a little controversial - but I think what you need to create is a Wiki.
If you want to "
make it available online as a collaborative open project" then putting it in Scribus is probably not a good idea. Scribus can be difficult to learn from scratch so people may be put off from joining in. Also, it's difficult to collaborate on document creation using Scribus - it's just not set-up for it. And, a PDF is static once created - except for individual annotations - and as such can't be "fixed" when changes are necessary.
You don't need to use any special software (MediaWiki etc.) to create a Wiki. All you need is some space you can give people read/write access to and some basic HTML/CSS.
Google Drive with Editey
http://www.editey.com/ looks like a good, free, combination that could be useful as a collaboration space but, of course, others are available.
HTML - as you probably already know - is easy to pick up, so the people collaborating on the project will be up-to-speed quickly and it's a standard markup that is very well documented. If you use a sensible file naming convention then linking between pages shouldn't be too hard.
You can get perfectly functional and easy to use CSS that works out of the box from loads of places. I'd personally recommend Foundation
http://foundation.zurb.com/ to get up-and-running quickly but that's a personal choice. You can then create a separate stylesheet for use when printing - if necessary - so people can read it on-screen or from a printed page and your content will be the same. (The "print" can always be sent to a PDF driver for physical distribution.)
I know this isn't a very "Scribusy" answer but I really wouldn't recommend using Scribus for what you are doing. It will take too long for contributors to get up-to-speed, you'll have problems with concurrent changes during editing, and your final output will be static.
Other solutions are available - Wiki software as already mentioned, Help file creators, Open/LibreOffice as mentioned by Nermander, and many others - but what I've suggested should be easy to implement and use. If you focus more on the content at the start it'll give you more time to look at alternatives later, if they're really needed.
Anyway, I hope this answer has been of some help and hasn't annoyed any of the Scribus community. I'm all for using Scribus - just look at how many posts I've written here - but only when it's the right thing to be using.