update solid concepts page to use less boilerplate
The main change is to use less boilerplate in the code snippets that describe inputs and outputs.
Users should only be using InputDefinitions and OutputDefinitions when they're needed.
- De-emphasizes solid reusability.
- Removes the examples, which were redundant with the examples in the explanatin sections. I think it would be helpful to later add examples that showcase somewhat more complicated functionality.
Test Plan: bk
Reviewers: schrockn, yuhan, owen, sashank
Reviewed By: yuhan
Differential Revision: https://dagster.phacility.com/D7386