Document the commands provided by the extension and corresponding key bindings

There are no comprehensive manuals that list the commands provided by the extension, their meaning and corresponding key bindings. So it's problematic to discover functionality of the plugin. When new user installs the extension he has few inconvenient options:

* Try to discover new and useful commands via `Command Palette` (`Ctrl+Shift+P`). There are many available commands, they haven't any docs and it is not clear which commands belong to the extension.
* Read the ["Features" page from the HIE docs](https://haskell-language-server.readthedocs.io/en/latest/features.html). It lists rather abstract features of the HIE itself, but doesn't mention how they connected to the VSCode extension. There are some demos but they cover advanced use-cases and don't mention basic commands like auto-completions, navigating, find usages, import identifier and so on.

Personally, it would be very helpful to me to have a separate documentation page listing the available commands, default key-bindings, their meaning and corresponding demos (if exists). For example, I spent a lot of time to discover that there are 'Haskell: import identifier' command that is capable to search identifiers not only in locally-installed modules but in the whole hackage database.