Feature: add MkDocs to build HTML and github actions to deploy the docs

[mlisovyi]

Main changes:

• The pull request implements Build HTML pages out of the README and host it on github pages #121
• Add mkdocs configuration to build HTML pages out of Markdown
  • the main landing page is the English version
• Add GitHub actions to buuld and deploy HTML into GitHub Pages automatically on each push/merge into master

Minor:

• dropped cross-references between translations to reduce maintenance burden. The links to individual translations (locally in the repo and externally in others) are all linked in the HTML docs in the left panel.
• Added a soft link to the main README in translations, as it is needed by mkdocs
• Downgrade all sections from level-1 to level-2 in order to include them in the TOC in the right pannel

Link to the documentation built in a fork
The final result looks like this (dark-mode version):

P.S. implementing it with a proper multi-lingual page would require more changes to the repository structure + would not allow to link external translations