feature symfony#4654 Rewritten from scratch the chapter about installing Symfony (javiereguiluz)

This PR was merged into the 2.3 branch.

Discussion
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Rewritten from scratch the chapter about installing Symfony

| Q             | A
| ------------- | ---
| Doc fix?      | yes
| New docs?     | yes
| Applies to    | all
| Fixed tickets | symfony#4122, symfony#4089

Commits
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7de83a3 Replaced a "note" by a "seealso"
b45c338 Imrpoved the wording of the note about using using legacy PHP 5.3 version
40d7772 Reworded the reasons why you should use Composer-based installation
2e72138 Wrapped a line to follow doc standards
7e9cd04 Display the same version number on Linux and Windows to avoid confusions
12c2557 Add an explicit command to better explain that we recommend to move symfony.phar to projects directory
12eb76e Reworded a confusing phrase
b665d86 Added a note about verifying Symfony digital signatures
8f202c6 Re-added a wrongly deleted link reference
c291a73 Rewritten from scratch the chapter about installing Symfony

Author: weaverryan