Docs: Create a reconversion guide

[jodator]

Suggested merge commit message (convention)

Docs (engine): Create a reconversion guide. Closes #8174.

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Additional information

• I've rewritten the sample several times and I think that the current content example is useful.
• The general idea is to have two parts of the guide:
  1. introduction to reconversion and reconversion concepts
  2. an example with the most relevant code is explained (the rest is in the full source code)

TODO:

• The initial section about the reconversion needs expanding.
• The example implementation needs closure with a full code.

[godai78]

I hereby approve this deep-dive guide to be worthy enough to join the great documents family. Providing the code is provided at the end.

[jodator]

OK, I've pimped-out the demo slightly (UI button to insert a card).
The code was added at the end and code examples were updated accordingly.

[AnnaTomanek]

I fixed a few things here and there, including a leftover ComplexInfoBox model element and some paragrahps 😉  Good job overall!

Things that we could still consider adding (although they can also be added as a later update):

• Replacing the yummy lorem ipsum with some real text (@godai78, any ideas?)
• Adding some "Next steps" section (or another form) with links to some other conversion guides (if it still makes sense) - our users do not always read the docs like a book, one after another, they will land here e.g. directly from the blog post.
• Adding the "select all" or whatever it is called UI element to the card that would allow you to select it and delete - now it's a bit counter-intuitive that you can add it and it's not really clear what you need to click to select it.

[AnnaTomanek]

Looks great - thanks for the tutorial, @jodator, and kudos for the demo content, @godai78 👏