Update .gitignore example

As confirmed by WouterJ on Stack Overflow [1], the example
.gitignore file [2] is not up-to-date.

This commit updates the documentation to reflect the current state
of the .gitignore file that is included in the Symfony Standard
Edition distribution [3].

[1] http://stackoverflow.com/q/23437768/1001110
[2] http://symfony.com/doc/current/cookbook/workflow/new_project_git.html#initial-project-setup
[3] https://github.com/symfony/symfony-standard/blob/master/.gitignore
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.
Indent the tip to be on the same level as the list item
Replace numbered list items with #

...so we don't have to renumber them every time we remove/add a new
item.
Small change in the wording

...as suggested by weaverryan.
Replace static version constraint (2.4.4)

...with a smarter one. Using Composer's tilde operator (~), also known
as the next significant release constraint, we instruct composer to use
at least Symfony 2.3, but not 3.0 or higher.
Some refactoring of the article

Shorten the 2nd list item a bit, and move it into a 'Tip' seems to make
more sense.
Small improvement
Remove dash
Update the tip about the global .gitignore
bug #3827 Update 'How to Create and store a Symfony2 Project in Git' (nicwortel)

This PR was merged into the 2.3 branch.

Discussion
----------

Update 'How to Create and store a Symfony2 Project in Git'

This pull request updates the cookbook article [How to Create and store a Symfony2 Project in Git](http://symfony.com/doc/current/cookbook/workflow/new_project_git.html#initial-project-setup).

It started with [this question (and its answer by Wouter J) on Stack Overflow](http://stackoverflow.com/q/23437768/1001110).

| Q             | A
| ------------- | ---
| Doc fix?      | yes
| New docs?     | no
| Applies to    | all (or 2.3+)
| Fixed tickets | n/a

Commits
-------

3f3d886 Update the tip about the global .gitignore
1baf7e0 Remove dash
6fd5e52 Small improvement
d9f72f9 Some refactoring of the article
083c1f5 Replace static version constraint (2.4.4)
32cee81 Small change in the wording
311f14b Replace numbered list items with #
6a19628 Indent the tip to be on the same level as the list item
34d61eb Update the complete 'Initial Project Setup' paragraph
011e0f0 Update .gitignore example