map-extent documentation

[AliyanH]

documenting changes in relation to Maps4HTML/MapML.js#887 and adding to the already merged PR #135

• 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: - document that the query does not work if layer is hidden - to be documented after map-link dev. hidden layer containing a query throws exception MapML.js#904

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. no longer an issue
• 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: - Should be added after it is finalized in the map-link PR work

[prushforth]

Looks good to me, merge when ready