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

[mfcovington]

"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.

[mfcovington]

I noticed that there are some new languages as pull requests. These will undoubtedly have issues with the CSS changes I've made. If it is OK, I will keep the changes I made, but add back the block-checkout class, too. In the future, the block-checkout class should probably be able to be dropped.