feature #51771 Update the design of the Symfony Welcome Page (javiere…

…guiluz)

This PR was squashed before being merged into the 7.0 branch.

Discussion
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Update the design of the Symfony Welcome Page

| Q             | A
| ------------- | ---
| Branch?       | 7.0
| Bug fix?      | no
| New feature?  | yes
| Deprecations? | no
| Tickets       | -
| License       | MIT

This PR refreshes the design of the Symfony Welcome Page for Symfony 7.0. This is the page that you see when you create a new Symfony app and browse it without making any change to it.

The new design keeps most of the features of the previous design, including the random color each time you reload the page.

This is how it looks:

| Smartphone | Tablet | Desktop
| ---------- | ------ | -------
| ![new-welcome-page-smartphone](https://github.com/symfony/symfony/assets/73419/9c1d9b22-d07d-45e3-a633-b6c475f1d5db) | ![new-welcome-page-tablet](https://github.com/symfony/symfony/assets/73419/5758c2c2-d3bb-404a-9460-b15686e5dcd8) | ![new-welcome-page-desktop](https://github.com/symfony/symfony/assets/73419/8994328d-e062-467d-b42f-70567007b7de)

This is how it looked **before**:

| Smartphone | Tablet | Desktop
| ---------- | ------ | -------
| ![before-welcome-page-smartphone](https://github.com/symfony/symfony/assets/73419/7cdf0df6-fc4f-47e0-979b-658629205726) | ![before-welcome-page-tablet](https://github.com/symfony/symfony/assets/73419/5dddd213-0d89-4f05-9035-77c140d49d35) | ![before-welcome-page-desktop](https://github.com/symfony/symfony/assets/73419/14afa467-8137-4b08-a4b0-895f9681b580)

Commits
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5d30505 Update the design of the Symfony Welcome Page