Start documenting the validator api

[laceysanderson]

Issue #5

This PR will document the TripalCultivate Validator plugins and how they are used both for ourselves and to help anyone who may want to extend our modules.

• Create a category for developer docs. I decided to put them in an "Extending functionality" section underneath the "Administration" documentation.
• Create a "Validators" page including a very short intro and links to expected background understanding.
• Document "What are Validators?". I started this but ran out of time to finish. Additionally there is currently a placeholder for a screenshot that I did not have time to setup a docker to take.
• Document "How are validators used by a Tripal Importer?"
• Create an image describing the flow of validation and the interaction between the importer and validator instances.
• Document the existing validators, when you will want to use them, what setters are needed and what process* methods are available.
• Think more about what else we should document and extend this checklist

Testing

Now that we have a docker for the docs we can test this locally as follows:

1. clone this branch locally
2. open the clone in vscode and choose to "Reopen in container"
3. once the container has built, go to ports and open it up in your browser.
4. Now navigate in your local copy of the site to Administration > Extending Functionality and review the content on this page.
5. Next, navigate to the Validators section under "Extending Functionality" and review the content on this page.

I expect it to look something like this currently.