Tripal 3 migration preserving bio_data values docs

[dsenalik]

Draft docs to add to the "Migrating Chado" page step 12. Publish all of your content types...

12. If you want to preserve your existing bio_data entity IDs and URL aliases from your Tripal 3 site, you will need to export that information before publishing content on your Tripal 4 site. To do so, copy this utility program from your new Tripal 4 site to your Tripal 3 server
    tripal_chado/migration/export_tripal3_entity_mapping.php
    and then run it on your Tripal 3 server:
    drush php:script export_chado_entity_mapping.php tripal3_entity_mapping.tsv
    and then either copy this data file back to your Tripal 4 server or docker container, or to your desktop computer.

13. When you publish, specify this data file in the "Tripal 3 Migration Options" section.

For step 11. add a "Note" box to not add the type field