Update contributions guidelines for docs

[blackbaud-johnly]

Update contributions message for docs to explain that we recommend (and appreciate) for developers to create documentation for their component because documenting it can be useful in uncovering issues with the component. And link to the docs repo. But contributing the docs is no longer a requirement. If they don't submit docs, we will document the component for them. However, need to stress that the demo is a requirement.

[blackbaud-johnly]

Addressed in #1429.