feat: Add ability to assign different exercises to each student automatically

[Rui00Barata]

• Kind: feature

• Close #531

Description

If you are a teacher with a large number of students and want to assign a different set of exercises to each student based on their level and exercise difficulty, this can be a time-consuming process.

With this feature, you can save time and effort when assigning exercises to your students on the Learn-OCaml platform.

Checklist

• [x] Read the CONTRIBUTING.md guide and:
  • [x] Use Atomic Commits so each commit gathers a single logical change
  • [x] Use Conventional Commits regarding commit messages (needed by our release toolchain)
• [x] Add/update documentation

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[erikmd]

Thanks @Rui00Barata for your contribution!

We started discussed with @AltGr about your PR (just briefly for the time being), to sum up at this point:

• it is not very clear for us if the implemented feature is already doable with the current learn-ocaml.master (namely, it is already possible from the teacher dashboard to assign specific exercises to a subset of students, e.g., filtered by tags)
• some parts of the PR looks not-enough documented, typically like this fragment:

  let p = [|[|1;1;1;2;2|]|]
  let a = [|[|1;1;2;2;2|]; [|1;1;2;2;3|]|]
  let e = [|[|2;2;3;3;3|]; [|2;2;2;3;3|]|]
  

  e.g. what is the exact semantics of each array?, why choosing array instead of mere lists?
  and also, one may want to make these settings configurable.

Anyway at this point, I guess @AltGr and us will try the branch soon; look more closely at the code; and return to you of course.

Thanks!