Consistency and grammar fixes; some rewording.

Good stuff!

I went through most of the manual and fixed mostly grammar and style (e.g. officially it's "GitHub" not "Github", "Git" not "git", etc.).

I also tried rephrasing some things that appeared awkward to me. Feel free to pick and choose the ones you like.

I expanded on a few sections that were overly specific to websites. However, I didn't touch the Security section, which I think needs a bigger overhaul. I'll file a bug for it.