feat: update Spring Boot project and documentation (#4383)

* Update Spring Boot project and documentation

* Updated images with new IntelliJ IDEA UI
* Update tutorial content as per the feedback by Sebastien Deleuze
* Restructured the code samples: all classes are now in separate files.

* chore: update screenshots

* Added fine names to the code samples and a few minor corrections in the text.

* trying to resolve merge conflict. discovered missing blocks of code.

* trying to resolve merge conflict (again)

* still trying to fix the merge conflicts

* more corrections

* missed the collapsible block

* update: first round of review

* MessageService code snippet should be simpler in at first

* MessageService snippet update

The `save` method in the database handling class should return the `Message` instance after insertion.

* update: TWr review

* update: add better quality pics

* fix: last fixes

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Co-authored-by: Andrey Polyakov <[email protected]>

Author: antonarhipov