Move Design Docs > In Progress > Entities + Fields to Extending Tripal

[laceysanderson]

Move Design Docs > In Progress > Entities + Fields to Extending Tripal.

[laceysanderson]

We will want to update these docs to match the final design which was implemented and include information on some methods + classes of particular importance. A brief example can be found in https://tripaldoc.readthedocs.io/en/latest/dev_guide/module_dev/routing.html and https://tripaldoc.readthedocs.io/en/latest/dev_guide/biodata/tripaldbx.html.