[TM-301] ManagedLedger doc

Description

Problem: we need a sample of automatic documentation capability, and it's generally good to have this core contract documented.

Solution: add automatic documentation generation.

I do not include documentation for Proxy contract yet.

Related issue(s)

https://issues.serokell.io/issue/TM-301

✅ Checklist for your Merge Request

Related changes (conditional)

• Tests (see short guidelines)

  • If I added new functionality, I added tests covering it.
  • If I fixed a bug, I added a regression test to prevent the bug from silently reappearing again.

• Documentation

  • I checked whether I should update the docs and did so if necessary:
    • README
    • Haddock
    • docs/

Stylistic guide (mandatory)

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• My code complies with the style guide.