Implement user guide for custom data validators feature

[jathanism]

Proposed Changes

Add a user guide for using the Custom Validators feature.

Justification

There is not currently a user guide and we want to have guides for all of the advanced features including examples and common usage patterns. Currently, this is buried under "Developing Plugins" > "Implementing Custom Validators. While it does give a code example, this feature especially could benefit from examples of how this code is used in practice, common issues, how to test that it is working, etc.