Updated installation instructions to use the new Symfony Installer
Restored the original section title
Fixed section headers
feature #4580 Updated installation instructions to use the new Symfony Installer (javiereguiluz)

This PR was merged into the 2.3 branch.

Discussion
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Updated installation instructions to use the new Symfony Installer

| Q             | A
| ------------- | ---
| Doc fix?      | yes
| New docs?     | yes
| Applies to    | all
| Fixed tickets | -

Commits
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f58db71 Fixed section headers
4b8e805 Restored the original section title
b69a5cd Updated installation instructions to use the new Symfony Installer