feat: add a new example

[xXenvy]

This pull request introduces a new example to the lib. The purpose of this demo is to illustrate the automatic generation of setter and getter methods for private fields.

[JhnW]

Maybe change example name (folder) to getters _generated or, even better, accessor.

[JhnW]

Please check doc_src/demos/ folder and please add your own doc for demo. From what I remember, just adding a file is enough, but if necessary, check the local documentation generation.

[JhnW]

Please check why output does not generate comments with correct spacing.

[xXenvy]

Please check doc_src/demos/ folder and please add your own doc for demo. From what I remember, just adding a file is enough, but if necessary, check the local documentation generation.

Done.

[xXenvy]

Maybe change example name (folder) to getters _generated or, even better, accessor.

I changed to accessors_generator. In my opinion, it is ok.