Verify CSS styles and the icon of the homepage link button

[IgnatBeresnev]

#3235 added a homepage link with an icon to the HTML format, as per #2948.

The current icon is a royalty-free placeholder, so we need to see if there's a preferred icon-pack within JetBrains / Kotlin, and change the icon if needed.

The styles also need to be verified, as they were mostly made to just work. There might be issues with scaling and/or the mobile layout, but we lack the expertise to be certain.

[IgnatBeresnev]

Grooming notes:

• There is no documentation on how to use this feature yet, so I'll need to show how to "activate" it, which will take ~15 minutes and can be async (in Slack)
• Need to choose the new icon for the home link
• Need to verify CSS styles, make sure they are mobile friendly and the icon is where it's supposed to be
• The icon is customizable by the users (comes by default in Dokka), so custom icons need to scale properly.
  • We can give guidance to our users when it comes to aspect ratio or even ideal size of the icon, but it should be reasonably flexible.
• Need to onboard Elena how to customize the icon to test it

[atyrin]

Customization is possible with such steps

1. Add a custom icon to a dokka project (e.g. to css/homeIcon.svg)
2. Customize icon name in custom css file (e.g. css/homeIcon.css)

.navigation-controls--homepage {
    background-image: url("../images/homeIcon.svg");
}

3. Pass custom asset and style sheet to dokka in the build script:

pluginConfiguration<org.jetbrains.dokka.base.DokkaBase, org.jetbrains.dokka.base.DokkaBaseConfiguration> {
        customAssets = listOf(file("css/homeIcon.svg"))
        customStyleSheets = listOf(file("css/homeIcon.css"))
}

N.B. do not forget about homepageLink field to add a button to navigation bar.