Configurations docs improvements

[interurban]

Quality and clarity improvements to https://www.pulumi.com/docs/concepts/config/

Resources

• Config improvements doc

### Tasks
- [ ] https://github.com/pulumi/docs/issues/11282
- [ ] https://github.com/pulumi/docs/issues/11283
- [ ] https://github.com/pulumi/docs/issues/11403

[cnunciato]

Ran through these docs myself and collected a bunch of concrete suggestions for us to consider: https://docs.google.com/document/d/1wPc-JC9oA_BIGX3n18T2aJsD_EEO9azvurVTZbGCgR8/edit

[jkodroff]

All the linked tasks are done. Need to look at the doc now and see if that's the actual list of things to do. If so, will update the description here or create new issues.