Fix chapter order and add 'keycloak.preferred_username' option

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 100.00%. Comparing base (aa71dbe) to head (f0bdce9).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #109 +/- ## =========================================== Coverage 100.00% 100.00% Complexity 47 47 =========================================== Files 5 5 Lines 138 138 =========================================== Hits 138 138 ```

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[robsontenorio]

Thanks !