Haddocks on Github Pages

[teodanciu]

Description

This PR introduces a new github workflow, triggered on master, which:

• builds haddock for every package, aggregating them as a site (listed on the landing page)
• deploys this site to github pages, so the haddocks will be accessible here, after the PR is merged: https://input-output-hk.github.io/cardano-ledger (replacing the current content, which seems obsolete anyway).

The links between the packages are working well (for example: when inspecting rules, clicking on anything from small-steps or cardano-core will take you there).
I'm unsatisfied for not being able to link to hackage the packages for which hackage documentation exists (like base). I was able to do this only at the cost of the links between packages not working, but I thought it's more important to have the latter working.

This is a copy of the site with this PR on my own fork, to demonstrate how the pages will look like: https://bienpulenta.github.io/cardano-ledger/

Checklist

• Commit sequence broadly makes sense and commits have useful messages
• New tests are added if needed and existing tests are updated
• Any changes are noted in the CHANGELOG.md for affected package
• The version bounds in .cabal files are updated
• Code is formatted with fourmolu (use scripts/fourmolize.sh)
• Cabal files are formatted (use scripts/cabal-format.sh)
• hie.yaml has been updated (use scripts/gen-hie.sh)
• Self-reviewed the diff

[lehins]

This is really awesome! Thank you.

It might be a good idea to add a link in the README.md to the landing page with haddocks.

[JaredCorduan]

❤️ ❤️ ❤️