Add docs task and workflow

[Patitotective]

Nico is a really cool game framework and I really like though something I sometime miss is up-to-date documentation, API.md is great but not all procedures are listed and I have to search through the source code, so I thought it would be nice to generate and host the documentation that nim doc generates.

• The docs task runs nim doc ... and creates an index.html file that redirects to nico.html.
• The docs.yml workflow uses https://github.com/peaceiris/actions-gh-pages to deploy the docs folder to gh-pages.

Thanks for making Nico. :heart:

[Patitotective]

Though I think to set up the gh-pages site you have to go to settings/pages and select the gh-pages branch. It would also be nice to set nico's repo link to https://ftsf.github.io/nico (not sure about the link).