Document the new Asset component + framework changes

[javiereguiluz]

The new Asset component has been merged in symfony/symfony#13234.

Todos:

• Document the component (Added the documentation for the new Asset component #4987)
• 1. Changes in configuration
• 2. Changes in Twig templates
• 3. Other changes

1. Changes in configuration

# OLD configuration
framework:
    templating:
        assets_version: 'v5'
        assets_version_format: '%%s?version=%%s'
        assets_base_urls:
            http: ['http://cdn.example.com']
            ssl:  ['https://secure.example.com']
        packages:
            # ...

# NEW configuration
framework:
    assets:
        version: 'v5'
        version_format: '%%s?version=%%s'
        base_path: ~
        base_urls: ['http://cdn.example.com', 'https://secure.example.com']
        packages:
            # ...

2. Changes in Twig templates

{{ asset() }} function is the same as before, but now it only allows to set two arguments:

{# Common case: no package name #}
{{ asset('logo.png') }}

{# When using packages #}
{{ asset('logo.png', 'images') }}

This means that absolute and version arguments are no longer available:

{# BEFORE #}
{{ asset('logo.png', absolute = true) }}

{# AFTER #}
{{ absolute_url(asset('logo.png')) }}


{# BEFORE #}
{{ asset('logo.png', version = 'v5') }}

{# AFTER #}
{# relative URLs - do nothing (version is automatically appended) #}
{{ asset('logo.png') }}

{# absolute URLs - get the version with asset_version() and append it manually #}
{{ absolute_url(asset('logo.png')) ~ '?' ~ asset_version('logo.png') }}

3. Other changes

• The asset*() functions are now defined in the Twig Bridge instead of the Twig Bundle.

4. Lastly, I have a question when using Asset component in a Symfony application: How can I use a custom version strategy in a Symfony application?

[xabbuh]

Thank you for taking care of this @javiereguiluz

[xabbuh]

The component's readme can be used as a starting point for this.

[xabbuh]

Reopening as I think we still have to cover the other three items.

@wouterj Please let me know if you disagree.