Schema errors after D11 upgrade

Problem/Motivation

When saving the settings form, an error message is displayed: No schema for simple_styleguide.styleguidesettings. These errors mean there is configuration that does not comply with its schema. This is not a fatal error, but it is recommended to fix these issues. For more information on configuration schemas, check out the documentation.

Steps to reproduce

- Upgrade to D11
- Visit /admin/config/styleguide/settings
- Click save

Proposed resolution

Create missing schema file. Patch is included.