WIP update documentation

[vilinski]

after build update, amazing fsdocs tool was able to produce html pages for all markdown files and API reference with search index, without any errors! (well almost)
it produces output folder with the documentation content, which can be pushed to appropriate place

• add logo.png file to package to the nuget
• add instructions for documentations build
• remove FsiRefGen git submodule
• add proper action to push the output folder content to github pages
• fix logo image (if the problem is not only local)
• add styling and/or index.html template?

[haf]

Great! Why remove the submodule though?

[vilinski]

The essence of this PR is to replace FsiRefGen with fsidocs
I still need to figure out what the FsiRefGen actually does, because I can't run it on my mac. But if it doesn't anything more than converting markdowns to html and generate API reference, then it is better to replace it with fsdocs. It should remove the need to manage a git submodule in a repo and to manage an external code in order to make Hopac releases in the future.