Alerts with dark background

[MewenLeHo]

Linked to #917

[julien-deramond]

OK for the documentation and the normal example with a .bg-dark.
Some fixes need to be applied (no need to display it in the documentation):

• Additional content doesn't work with a .bg-dark: the links are displayed in black.
• Dismissing doesn't work with a .bg-dark: close button is displayed in black (when hovering as well); it is maybe linked to the close button implementation with a dark background

[MewenLeHo]

Done

[julien-deramond]

With the merge of the dark texts PR, IMO the only content of this PR should be:

### Dark background

Alerts come with dark variants just by adding a `.bg-dark`.

{{< example class="bg-dark" >}}
<div class="alert alert-success bg-dark" role="alert">
  <span class="alert-icon"><span class="visually-hidden">Success</span></span>
  <p>Success notification text goes here.</p>
</div>
<div class="alert alert-success bg-dark" role="alert">
  <span class="alert-icon"><span class="visually-hidden">Success</span></span>
  <div>
    <h4 class="alert-heading">Success notification text with <a href="#">a link</a> goes here.</h4>
    <p>Description text with <a href="#">a link</a> goes here.</p>
  </div>
</div>
<div class="alert alert-warning bg-dark alert-dismissible fade show" role="alert">
  <span class="alert-icon"><span class="visually-hidden">Warning</span></span>
  <p>Warning notification text goes here.</p>
  <button type="button" class="btn-close btn-close-white" data-bs-dismiss="alert"><span class="visually-hidden">Close</span></button>
</div>
{{< /example >}}

[louismaximepiton]

Minor fix ? Sounds great to me !

[julien-deramond]

Sounds good to me. @louismaximepiton anything to add or can I merge?