If you are familiar with them, this is a bit like Microsoft Word stylesheets, only vastly more powerful. I found the best way to learn was to read the source to lots of example documents, seeing how other authors had written similar information. The conversion process is not that simple. That's changing, and in particular there's a project underway to convert the documentation to DocBook. Yes you can.
Any documentation is better than no documentation.
If you've got some documentation to contribute and it's not marked up in LinuxDoc or DocBook, don't worry. Submit the documentation as normal. Someone else on the Project will grab your committed documentation, mark it up for you, and commit it. With a bit of luck they'll then send you the marked up text back. Obviously, this slows down the committing process, since your submitted documentation needs to be marked up, which may take an evening or too.