[Inkscape-docs] First contribution

Here it is. I mostly did some basic work with the intro document. I significantly changed many things, hopefully both keeping the main concepts as well as making them more concise and up-to-date.

I've begun to change some of the linkend names to use dashes, so I'm sure there are broken links caused by me. I'll get to them as I continue to work.

If it looks good, can someone please contribute it for me?

Also, please add this screenshot to sshot.en.

Thanks.

JF