[docs] Refactor for MDBook and Setup Pages

Per @pluto-9r / @GrassRootsCrypto request this makes some tweaks to developer docs for compliance with https://github.com/rust-lang/mdBook and adds a pipeline step to generate them and update the GitLab Pages. I triggered a manual pipeline for this so the generated site can be viewed here (someone with DNS access can update dev.thorchain.org and add the certificate): https://thorchain.gitlab.io/thornode/.