Add frontend css file for pages

[eerison]

Fixing the site's page to work as 3.x

I am targeting this branch, because the bootstrap wasn't loading in site's page.

Closes #1573.

Changelog

### Added
- Added `assets/css/frontend.scss` to use in site's page

To do

• Fix pipelines
• Update the documentation;

[eerison]

Hey @jordisala1991 to make this accessible from my project, I guess I should run some build here, don't you?

[eerison]

I remember some similar issue related with admin bundle

[eerison]

any idea why it's not loading ?

[eerison]

Ok I found the issue

it need to be added here

sonata_page:
    assets:
        stylesheets:
            - bundles/sonatapage/frontend.css

[eerison]

Now it's look nice again :)

[eerison]

[eerison]

Can you review @jordisala1991 and @VincentLanglet

[eerison]

it worked with default configuration

[eerison]

Ok now it's loading only frontend.css :)

[eerison]

could you check again @jordisala1991

[jordisala1991]

We should also deprecate the css and js config on 3.x and remove it on 4.x after (other Prs)

[VincentLanglet]

We should also deprecate the css and js config on 3.x and remove it on 4.x after (other Prs)

Can you add it to the todo @eerison ?