Simplify Gradle layout, + KNIT, - ANK and fix site code fences

[raulraja]

This PR is a joint effort with @JavierSegoviaCordoba and @nomisRev.

1. Simplifies the gradle setup into a single project with all the modules, still preserves the folder layout.
2. Moves a lot of the gradle config into https://github.com/arrow-kt/arrow-gradle-config
3. Replaces Ank in favor of Knit and fixes all the docs broken snippets.
4. Works around many dokka bugs where Dokka is incorrectly rendering markdown and code fences. For example, everything processed by DokkaGFM was returned as HTML escaped strings.
5. Removes the need to have a build/_site which is where Ank was postprocessing the docs folder. @franciscodr We need you to please review the publishing workflow as the nesting in the docs does not exist anymore and most likely all steps related to removing the index file and copying the different versions need paths adjustments. I did my best but I don't understand how that works and I have no way to test it. Before this gets merged we need this final commit by @franciscodr ensuring the publishing of the site is going to work.

[franciscodr]

@raulraja @nomisRev You can check how the site will look in this link

[nomisRev]

LGTM

[i-walker]

👍🏾 Nice

[franciscodr]

🚢