Provide ability to specify a custom favicon for the HTML output

[mahozad]

Created this issue because it was suggested in #2513.

A custom favicon (which may be different from the header logo) cannot be provided for the output HTML format.

A workaround is to name the favicon logo-icon.svg and specify it as a custom asset in the configurations:

tasks.dokkaHtml.configure {
    dokkaSourceSets {
        pluginConfiguration<DokkaBase, DokkaBaseConfiguration> {
            customAssets = listOf(
                file("docs/logo-icon.svg"),
                // ...
            )
        }
    }
}

Some problems with this is that I may want to name my favicon something other than logo-icon.svg and may want to use a format other than SVG (for example, PNG).

Are you willing to provide a PR?
Maybe (depends on if the maintainers want it and if I can).

[solrudev]

Use case: prefers-color-scheme CSS media query for monochrome white/black logo. If browser highlights a tab with white background (prefers-color-scheme: light), favicon should turn black, but header logo should be white.

[solrudev]

There's a workaround to customize favicon and header logo separately.

If you need to provide custom header logo different from favicon, logo-styles.css should be configured which links to a different logo image. Favicon won't be replaced with the logo you provide in the CSS and will still be taken from logo-icon.svg in customAssets.