[2.0] Document a compatibility mapping

[strawgate]

We should document an optional component template that allows users who built content for <0.70 to seamlessly use those visualizations and dashboards with 2.0.

[strawgate]

From my testing we'll need to include:

hass.entity.attributes -> hass.attributes
hass.domain -> hass.entity.domain
hass.entity_id -> hass.entity.id
hass.entity_id_lower -> hass.entity.id
hass.object_id -> hass.entity.object_id
hass.object_id_lower -> hass.entity.object_id
hass.value -> hass.entity.value
hass.entity.attributes.friendly_name -> hass.entity.friendly_name

missing fields:
hass.entity.device.id -> #423

[strawgate]

It looks like maybe you cannot actually create an alias to a multifield so this may not be possible in the way I thought it might be