Add documentation to `Lorentz.Ext`

Description

Problem: this module is not documented.

If we are going to bring Lorentz to the masses, it would be nice if there is no need for people to guess the meaning of the methods or find it anywhere else.

Also, as my personal preference I'd like to outline that in stackType it is possible to omit non-significant parts of type, this feature seems to be underestimated.

Solution: add documentation.

Related issue(s)

Resolves #

✅ 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:
    • Root README and other README.md files
    • Haddock
    • docs/
  • I updated changelog files of all affected packages released to Hackage if my changes are externally visible.

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