Update the complete 'Initial Project Setup' paragraph

As proposed by WouterJ in https://github.com/symfony/symfony-docs/pull/3827
this commit rewrites the 'Initial Project Setup' paragraph.

The documentation now describes the recommended way of creating a
new Symfony2 project, using Composer to download the Standard Distribution
+ vendors (instead of downloading the zipped version). As the Standard
Edition already contains a .gitignore, the example is replaced by a
link to the one stored at GitHub. The article also explains which files
are excluded by .gitignore.

The last step (downloading vendor libraries using Composer) is removed
as this is now covered by step 1.