[#307] Fix links in READMEs

Description

Problem: Hackage's handling of relative links and anchor links is broken.

Solution:

• Relative links to modules now point at the corresponding haddocks on Hackage.
• Relative links to other files are turned into absolute links to the GitLab repo.
• Anchor links are turned into absolute links to the GitLab repo.

These are the packages whose READMEs need to be checked:

• code/cleveland/cleveland.cabal
• code/cleveland/examples/edsl-demo.cabal
• code/indigo/indigo.cabal
• code/indigo/tutorial/indigo-tutorial.cabal
• code/indigo/tutorial/standalone/myproject/myproject.cabal
• code/lorentz/lorentz.cabal
• code/morley-client/morley-client.cabal
• code/morley-debugger/morley-debugger.cabal
• code/morley/morley.cabal
• code/morley-multisig/morley-multisig.cabal
• code/morley-prelude/morley-prelude.cabal
• code/tasty-hunit-compat/tasty-hunit-compat.cabal

Related issue(s)

Resolves #307 (closed).

✅ 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.

Stylistic guide (mandatory)

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