[2.4] Document ExpressionLanguage component

[wouterj]

The ExpressionLanguage component is the biggest feature for 2.4
but it dont' have any docs yet. It can be split in several PRs:

The standalone component

• Introduction
• Syntax
• Adding custom functions (the extension point of the component)
• Using ParsedExpression and SerializedParsedExpression

Framework integration

• Routing integration
• Security integration
• Service Container integration
• Validator integration
• A cookbook entry that simply highlights these 4 usages and links to the individual spots in the docs where these are explained more

More information in the great pr description of symfony/symfony#8913

[wouterj]

I'm going to work on the standalone component.