hints.md: adjust default URL and provide start command for qt-kiosk-browser

This MR is:

• just a second part of https://secogroup.atlassian.net/browse/YT-74 task;
• adjusts a default URL for qt-kiosk-browser in hints.md
• describes how to start qt-kiosk-browser manually after boot

The first part, which adds a service for qt-kiosk-browser is handled in following MR: seco-ne/yocto/layers/meta-seconorth-distro!747 (merged)