map-link documentation

[AliyanH]

To be worked on along with Maps4HTML/Web-Map-Custom-Element#916

• bundle up the rel attribute documentation in associated groups, for the ~7 different type of uses for map-link, i.e. used as templated layer, legend, stylesheet, etc.
• for the attributes of map-link, we should specify for which rel="", the associated attributes work. For ex. for projection attribute, it is to be only used when rel="alternate"
• TBD