Adjusting image sizes and improving text readability

I made two principal changes here.

First, I replaced several images on the "Installation with WebMatrix" page. These are images I originally created, but then realized exceeded the maximum display size as posted in the Documentation style guidelines. I reduced the image sizes and re-loaded them.

Second, I improved the readability of both the "Installation with NuGet" and "Upgrading" sections. I did this by improving grammar and adding bold emphasis to key terms and phrases.