Convert BLS12381 haddock examples into doctest

[dancewithheart]

Fix #647

Description

Converts the BLS12381 PoP aggregation Haddock example into an executable doctest.

Tested with:

bash -x scripts/doctest.sh
echo $?

output shows:

***** cabal doctest cardano-crypto-class:lib:cardano-crypto-class *****
+ cabal repl --with-repl=doctest '--repl-options=-w -Wdefault' --build-depends=QuickCheck --build-depends=template-haskell cardano-crypto-class:lib:cardano-crypto-class
...
Examples: 34  Tried: 34  Errors: 0  Failures: 0

and last command exit code is 0

Checklist

• Commit sequence broadly makes sense and commits have useful messages
• New tests are added if needed and existing tests are updated
• All visible changes are prepended to the latest section of a CHANGELOG.md for the affected packages.
  New section is never added with the code changes. (See RELEASING.md)
• When applicable, versions are updated in .cabal and CHANGELOG.md files according to the
  versioning process.
• The version bounds in .cabal files for all affected packages are updated.
  If you change the bounds in a cabal file, that package itself must have a version increase. (See RELEASING.md)
• Self-reviewed the diff