Add directive test for admonition/danger

[smnandre]

Follows this topic about admonition levels on symfony/symfony-docs: [RFC] Differenciate "caution" admonitions #19181

The bloc danger will be used soon in Symfony docs (cc javiereguiluz comment ).

So this PR allow a single test to check its compilation.

[smnandre]

Failing test seems related to Doctrine Parser, i suggested a fix

[javiereguiluz]

Thanks Simon!

In order to "demote" the current caution admonition, we should update its icon.

In this PR, the new danger is using the same icon as the caution. That's fine.
But, let's change the icon of caution to use the typical triangle icon.

Our icons come from the project Heroicons (https://heroicons.com/outline) so let's use this for caution:

<svg xmlns="http://www.w3.org/2000/svg" fill="none" width="24" height="24" viewBox="0 0 24 24" stroke="currentColor"><path stroke-linecap="round" stroke-linejoin="round" stroke-width="2" d="M12 9v3.75m-9.303 3.376c-.866 1.5.217 3.374 1.948 3.374h14.71c1.73 0 2.813-1.874 1.948-3.374L13.949 3.378c-.866-1.5-3.032-1.5-3.898 0L2.697 16.126ZM12 15.75h.007v.008H12v-.008Z" /></svg>

Thanks!

[smnandre]

Done!

I reused the triangle icon already present in the file
(the light code difference is due to -i think- those already in the project were compressed a tiny bit)

[javiereguiluz]

Thanks Simon for working on this feature, this is much appreciated.

[smnandre]

@javiereguiluz thank you people for all the work done here :)