Rework the main Readme

[MGaetan89]

Pull request

Description

This PR updates the main Readme file.

Changes made

• All badges are now from shields.io to have a consistent display.
• Provide an overview of Pillarbox's features.
• Add a section about the demo applications.
• Add a link to GitHub's documentation about adding a repository.
• Add links to each Pillarbox modules documentation.

Checklist

• Your branch has been rebased onto the main branch.
• APIs have been properly documented (if relevant).
• The documentation has been updated (if relevant).
• New unit tests have been written (if relevant).
• The demo has been updated (if relevant).
• All pull request status checks pass.

[github-actions]

Code Coverage

Overall Project	46.59%	🟢

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