Update Windows Installation Methods.

The Windows installation methods listed in the Getting Started guide don't mention a couple of recent developments. I updated them:

• They now list Steam and the Windows Store as potential (non-free) options, with clear indication that they are the same product, just auto-updating.
• They now indicate that the download page will recommend a download version.
• They reference the download page for getting the portable version, with the KDE download directory mentioned only as an alternate location.
• They have the correct Windows version requirement.
• The outdated Intel bug was removed.

Differential Revision: https://phabricator.kde.org/D18710