Upgrading docs to Symfony 4.0

[javiereguiluz]

This meta-issue lists all the big changes needed to upgrade the docs to Symfony 4.0:

• Replace kernel.root_dir by kernel.project_dir everywhere. Never explain kernel.root_dir but maybe add a note mentioning that project_dir didn't exist before 4.x and people used root_dir.
• The file extension of YAML files is now .yaml instead of .yml
• In YAML files, null values are represented as null instead of ~ (change was reverted)
• Application code is no longer stored in AppBundle or any other bundle. Bundles for own code are discouraged.
• Lots of components/bundles (e.g. Symfony Console or MonologBundle) are not installed by default. We should add notes telling users to do: composer req console, etc.
• We need to decide what to do with Symfony Flex aliases. For example, which command should we use to install the Symfony Console package? composer req cli, composer req console, composer require console, composer require symfony/console, etc.
• Most config options are disabled by default. For example, the session service is not available by default because session config is commented in framework.yaml
• See Remove Debugging Guide #8538 - make sure we mention all of the debugging commands somewhere
• Commands are now always services, being in a Command/ directory is not important
• remove references to symfony/framework-standard-edition
• web/ to public/ (also see Fix server configuration for Symfony 4 #8693 (comment))
• _controller -> controller

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Remove deprecated features:

• removed autowiring services based on the types they implement
• removed the DefinitionDecorator class, replaced by ChildDefinition
• removed the AutowireServiceResource class and related AutowirePass::createResourceForClass() method
• removed LoggingFormatter, Compiler::getLoggingFormatter() and addLogMessage() class and methods, use the ContainerBuilder::log() method instead
• removed FactoryReturnTypePass
• removed ContainerBuilder::addClassResource(), use the addObjectResource() or the getReflectionClass() method instead.
• removed support for top-level anonymous services
• removed silent behavior for unused attributes and elements
• Removed the "framework.validation.cache" configuration option. Configure the "cache.validator" service under "framework.cache.pools" instead.
• Support for stacked errors in the ErrorHandler is deprecated and will be removed in Symfony 4.0.

[javiereguiluz]

Closing because this is a too broad issue. We're already taking care of all these in multiple separate pull requests.

[weaverryan]

@javiereguiluz I'm going to re-open this. This IS indeed broad. But I'd like to either check all the boxes in the description or create separate issues. For example, we have NOT removed all references to AppBundle yet on 4.0. We're still working on that :)

[javiereguiluz]

We finally checked all boxes of this meta issue ... so closing it as fixed.