Document type guards for Data objects and Entities

[David Arenas]

This is for documenting changes done in frontity/frontity#449 which are summarized here: https://community.frontity.org/t/extend-sources-data-interface-with-types-from-new-handlers/1187/38

Basically, it would be to add docs for those type guards and explain their purpose. Maybe we need to create a new page for @frontity/source package (types and type guards are included there).

In addition, the isTaxonomy conditional tag is deprecated and I guess we would have to fix some parts already documented in @frontity/wp-source.