Move doc site to new Hyperledger template

[nguyer]

This PR updates the doc site to use the new Hyperledger recommended framework using mkdocs and GitHub Pages. It uses a plugin called mike to manage the versions

• GitHub workflows have been updated to use this new build as well
• All of this should fix the problem we always had when someone released a hotfix release for an older version of FireFly it messed up the docs site in the past, because the concept of latest was ambiguous
• I've backported the docs for one previous release v1.2.2 into the new framework so it shows up in the version list. If we feel like we need more than that, we can do additional releases, but it's a bit of manual work

New version of the doc site is live at: https://hyperledger.github.io/firefly/latest/

[nguyer]

Awesome stuff!  One small comment

Might be also worth adding a section to the README of the docs of how the documentation is versioned based on tags and how the head/latest works?

Yeah it is also on my mind that I need to update the docs on how to update the docs. We have a page in the Contributors' guide on that but it is no longer accurate now. Will push one more commit for that.