Update MVT documentation

[cazitouni]

Update related to QWC2 #289

[tpo]

@manisandro @cazitouni - shouldn't the declutter option be on by default? Is there a downside not to enable it by default?

[manisandro]

@tpo The is a potential performance penalty with decluttering enabled. OpenLayer disables it by default: https://openlayers.org/en/latest/apidoc/module-ol_layer_VectorTile-VectorTileLayer.html

[tpo]

@tpo The is a potential performance penalty with decluttering enabled. OpenLayer disables it by default: https://openlayers.org/en/latest/apidoc/module-ol_layer_VectorTile-VectorTileLayer.html

Thanks a lot for explaining @manisandro !