Change the wording of "Checkout a Repository" to "Copy a Repository"

**"Checkout a Repository"** might lead to some confusion for people learning git, since `git checkout` is an actual thing and is totally different from `git clone`. I've changed the wording to: **"Copy a Repository"**

I've made the appropriate changes to all index.XX.html (including renaming of an anchor and a class). There were three that already used 'copy' instead of 'checkout' and two (Korean & Chinese) that I didn't know how to translate.

I also fixed the css file, since `block-checkout` class was renamed to `block-copy`.