* 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.
Potential TODO - lead with an example that does not use config schema (just Any or something).