Fix #2277, update Style Guide page.

I've updated the style guide page to remove most instances of add-on or extension talk. I ended up removing the database portion, as well as some other text that didn't seem appropriate.

Because this page is kind of in between a guide to contributing and a guide to how to style your CI app's code, I'm not really 100% sure of its end purpose as it stands, so I didn't go too in-depth with the whole thing.