Upgrade Tripal Importer documentation

[laceysanderson]

We need to upgrade the Tripal Importer documentation. The original Tripal 3 documentation is here: https://tripal.readthedocs.io/en/latest/dev_guide/custom_data_loader.html

This will be moved under Extending Tripal > Biological Data Storage in the new docs.

The process is very similar with most of the changes being in the Static Variables which have moved into the annotation block. There are also some small changes in the form and form validate functions reflecting the move to OOP.

You can see examples of core importers here: https://github.com/tripal/tripal/tree/4.x/tripal_chado/src/Plugin/TripalImporter

[spficklin]

On it. Using branch tv4-issue60-importers

[spficklin]

Documention added with PR #61 but it still needs to include more details about functional testing.