[UX] Add help text and improve the UI to clarify things around layout templates

This is similar to #6309 but for the layouts UI.

While for most of us "chronic" Backdrop users certain things are obvious, they may not be that obvious to people new to our CMS. See discussion in this thread in Zulip for more context: https://backdrop.zulipchat.com/#narrow/stream/218635-Backdrop/topic/Flex...

Lets add some help text that a minimum indicates that if none of the available templates is suitable, more can be added either from the "Install new layout templates" tab (ready-made), or from the "Layout templates" tab (create new templates with custom number of rows rows and region widths).