Discuss redefining agent types in the docs

[Datseris]

In the main tutorial add an explicit top-level header "Redefining agent structs".

There we should discuss firstly, that this is not really a big deal. Just do mass rename in VSCode with Ctrl+Shift+H.

Then, we discuss that this is a fundamental limitation of the language, and not using structurs is simply a much worse idea due to plummeting impact on performance.

Lastlyu, say that we are crrently considering other options and cite #907 .