MDL-83541 Add qtype restore code documentation

[marxjohnson]

This describes the required updates to qtype plugins following to implementation of MDL-83541 to avoid duplication of questions during restores.

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[timhunt]

Thanks Mark. That looks like a really good new docs page.

The only bit I don't really like is the use of $name for the plugin name. Is this a convention used elsewhere in the docs? I managed to avoid it comletely on https://moodledev.io/docs/5.0/apis/plugintypes/qtype/newquestiondefaults. On https://moodledev.io/docs/5.0/apis/plugintypes/qbank/filters we use qbank_myplugin. Up to you to make a final call.

[andrewnicols]

I'd also second not making this a var. Another option would be to use a placeholder:

In your  `restore_qtype_<name>_plugin`  class, add an override of the `remove_excluded_question_data()` method, which removes this

[timhunt]

I like Andrew's suggestion. Feel free to updated qbank/filters while you are at it Mark (he suggested cheekily :-))