Use Xcode for developing web pages

No, use DashCode for developing iphone web apps. See Mobile Safari Web Application Tutorial for more info.

You can indeed write web pages with Xcode. I was able to do so by creating a new empty project, then creating a new file in it with a .html extension. Ready to go.

There is no real difference between webpages for normal browsers and iOS devices. Webpages for iOS devices may have special meta tags changing the viewport and rely on WebKit-specific things, but there's no significant difference.

If you already know HTML, you may want to start at http://developer.apple.com/safari.

As for dynamic web pages, yes, of course. Again, it's not that different from developing dynamic web pages for normal browsers.