Revamp the configuration concept section
- Explained the basic idea of config (you pass it to pipelines at runtime).
- Explained what situations users should use config in.
- Took out the config-mapping example, because I think it violates our guidance on when to use config (the Spark parameters that are the same across all runs shouldn't be config at all).
- Added in an example of how to make a config value available to multiple solids.
Test Plan: manual inspection
Reviewers: schrockn, yuhan, alangenfeld, sashank
Reviewed By: schrockn
Differential Revision: https://dagster.phacility.com/D7438