Document alternative Requirements Checker usage, perhaps add it as best practice

[Amunak]

This is a follow-up on a Slack discussion.

I think that the deployment docs should mention that the Requirement Checker can be used without the Symfony "binary", which is something most people probably don't want to deal with in production.

I would suggest to at least mention that there is an option to require symfony/requirements-checker, then adding "vendor/bin/requirements-checker": "php-script", as one of the first of auto-scripts in composer.json. (Note: there is a recipe that adds it there automatically, but in a different way that does not run it under the same PHP executable, being useless on setups with multiple PHP versions).

Even better, there is no reason I can think of why people shouldn't be doing this - the requirements checker can catch a lot of common mistakes (like forgetting to set up date.timezone) and it runs extremely fast. Which is why I suggest that it should also be added as a best practice.

[javiereguiluz]

I think you are right. We're trying to fix this in #14809. Thanks!