[UX] install.php: Provide help text for site name

Description of the need

During Open Source Day today, two participants both mentioned uncertainty over what was required for site name during installation. They weren't sure if there was something specific that was required in this field.

I think that having "localhost" here as default (when installing locally) is confusing.

Proposed solution

Add some help text for this field that says something to the effect of: "Your chosen name for this website, this can be changed later."

Alternatives that have been considered

Adding a default name (placeholder) that reads something like "My Site" or "My Backdrop Site." See: https://github.com/backdrop/backdrop-issues/issues/2291

I think that placeholder text might help demonstrate the intention for this field.