Simplify the page "Getting the Code" on WebKit.org

Simplify the page "Getting the Code" on WebKit.org
https://bugs.webkit.org/show_bug.cgi?id=113980

Patch by Benjamin Poulain <bpoulain@apple.com> on 2013-04-07
Reviewed by Sam Weinig.

* building/checkout.html:
Instead of interleaving instructions for Mac and Windows, just
copy the things that make sense and have two clear sections.

Also mention Linux, it is the exact same instructions as Mac OS X.

Make svn checkout the alternative to a download of the tarball. It is
almost impossible to do a svn checkout on slow networks nowadays.

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@147890 268f45cc-cd09-0410-ab3c-d52691b4dbfc