- the main command is =:h= which should open =help.txt= if given no arguments;
this should be enough if you want to explore much of Neovim itself as the document has an index of tutorials and local additions (most likely from the installed plugins)
- the help documents are written in a custom formatting language that is recognized as a separate filetype;
in addition to navigating the buffer like any other buffer, you can also run =gO= to show the table of contents