feat: add site favicon to skeleton.html

[furkankurt]

Django Unfold doesn't have a favicon feature.
We had to override the whole skeleton.html template to add a favicon. (#206)

I have added a site_favicon.html file to the unfold/helpers directory, we can add all favicon types to the website.
Instead of overriding the skeleton.html, we will override just the site_favicon.html.

PS: I added example favicon links as template comments.

[frobert]

I would suggest you also add option to change the favicon in settings.py UNFOLD config the same way as SITE_ICON, ex.

"FAVICON": { 
  "light": lambda request: static("favicon.png"),
  "dark": lambda request: static("favicon_dark.png"),
}

[lukasvinclav]

Hey, thanks for good initial PR. Would you mind to create an issue for this task? Don't forget to link this PR there.

[furkankurt]

Hey, thanks for good initial PR. Would you mind to create an issue for this task? Don't forget to link this PR there.

Hello, thank you for your valuable comment. I've created an issue about this PR.