[TBTC-16] Comments in markdown doc

Description

Problem: It would be nice to have an ability to add comments to the contract documentation in markdown, e.g. for licenses and such stuff.

Solution: Add DComment and its DocItem instance to render markdown comments.

Related issue(s)

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

✅ 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)

• My commits comply with the following policy.
• My code complies with the style guide.