New page (and perhaps sub-pages for): Styling the map

[Malvoz]

It might be a good idea to have a "Styling the map" page which would include explanations for:

• pre-styling to avoid FOUC (generally always recommended)
• using fallback content:
  • importance of the :defined pseudo-class
  • pre-styling to avoid Layout Shift (occurs because the browser has to wait for the element to be defined before applying height/width)
  • resetting display: none of fallback content if JS is disabled

And other - perhaps more common use cases - such as:

• responsive maps (max-width: 100%)
• prerequisite for % value height maps
• full viewport sized maps (without scrollbars! 😋)
  (see also Fix issue with standalone maps not always spanning the entire visible viewport experiments#93)

We could then link to that page from https://github.com/Maps4HTML/Web-Map-Custom-Element#styling-the-map-element (or remove that section altogether).