Make this repo as IOTAPay Documentation

IOTAPay documentation here instead of deprecating it.

For documentation, some viable options:
1. [Gitbook](https://www.gitbook.com/)
2. [Docsify](https://docsify.js.org)
3. [Hexo](https://zalando-incubator.github.io/hexo-theme-doc/) with [doc theme](https://zalando-incubator.github.io/hexo-theme-doc/)
4. [Docusaurus](https://docusaurus.io/)
5. [Hugo](https://gohugo.io/) with [documentation theme](https://themes.gohugo.io/tags/documentation/)
6. [Couscous](http://couscous.io/)
7. [ReadTheDocs](https://readthedocs.org/)

For hosting can use github pages for now. Also can use Netlify if required.