Moved "custom backoffice APIs" to tutorials + update code snippets

[kjac]

Description

I have moved the "custom backoffice APIs" article from the Fundamentals section to the Tutorials section, where I think it belongs 😄 (the article is currently here).

I have also updated the code snippets slightly:

1. Added {% code %} and {% endcode %} tags.
2. Ensured the "full implementation" snippet can compile on its own by including all classes/enims, the required using instructions and a namespace for the controller.

Lastly I have prepared for this tutorial to be expanded; we will be adding more advanced stuff later on 😄

Type of suggestion

• Typo/grammar fix
• Updated outdated content
• New content
• Updates related to a new version
• Other

Product & version (if relevant)

V14

Deadline (if relevant)

As soon as it can be 😄

[alina-tincas]

Hi @kjac thank you for the PR 🙌

That looks good 💪 I have just one suggestion, to make it more clear where you can find the end result (in the swagger- management API). I was also wondering if perhaps it would make sense to link from management API article to this example as well?

[alina-tincas]

Thank you accepting the suggestion, merging this in 😊 💪