Include custom distro text on first page

@ngraham Feedback from Akademy:

by not mentioning the distro at all on the first page, we make it less likely that the distro will adopt it, or more likely that they'll patch the first page to say something different

Some ideas:

• "Welcome to KDE Plasma!" could become "Welcome!" with more generic content, maybe followed by a "What is KDE Plasma?" page.
  • Be wary with adding more pages.
• A tagline for distros would be good for the first page, as well as on the live environment page, but this is not supported by /etc/os-release and falls under the purview freedesktop/systemd(?)
  • e.g. "Arch Linux, a lightweight and flexible Linux® distribution that tries to Keep It Simple.", "Fedora CoreOS is an automatically updating, minimal, container-focused operating system."

Quick concept:

Needs iteration to remove duplication in title and description.