Make it simpler to customize favicon

Thanks for the great theme.
Trying to customize the favicon I saw that it seem to be defined with:
```

```
in the base template.
This makes it not trivial to customize the favicon. Practically the site admin needs to customize the theme, using inheritance.

Are you interested in adapting the code to make it simpler for site admins to use their own favicon. If yes, I can write a PR for it.

Locate the base template containing the favicon link shown in the issue and review how Grav theme inheritance currently handles site customization. Define a simpler admin customization path, then verify that a site can use its own favicon without copying or inheriting the theme.

Written by the indexing model from the issue text.