minor #15924 [Mercure] sync docs with the last version of Mercure (dunglas)

This PR was squashed before being merged into the 5.3 branch.

Discussion
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[Mercure] sync docs with the last version of Mercure

Add docs for the features introduced in the latest version of Mercure and for the changes recently made in Symfony CLI.

Commits
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6644f4c [Mercure] sync docs with the last version of Mercure

Author: javiereguiluz