Sync/Document changes for map-extent custom element

[AliyanH]

documenting changes in relation to Maps4HTML/MapML.js#887

• add checked/hidden attributes where needed
• add complete element endings i.e. </map-extent> instead of <map-extent .... />
• map-extent label "when an extent does not have a label, it is hidden" is no longer true
  • mapml documentation for attributes, ex. checked="checked" is required for the parser
• Add documentation for hidden attribute for layer to reflect this code for query layers on MapMLLayer.js -> getQueryTemplate:

if (
        this._layerEl.checked &&
        !this._layerEl.hidden &&
        this._mapmlLayerItem
      ) {

• For our API's we learned that they can only be used after the custom element is attached to the DOM. This needs to be reflected on the web-map-doc. Found using the features API experiment.
• Document map-change event for layer- element.
• remove extentload + createmap (replaced with metadataload)
• Document the use of map-meta's inside of map-extents and how they work: