Improve secrets concept pages

[interurban]

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

Resources

• Concept review document

### Tasks
- [ ] https://github.com/pulumi/docs/issues/11276
- [ ] https://github.com/pulumi/docs/issues/11277
- [ ] https://github.com/pulumi/docs/issues/11320