Add documentation for all commands

I'm wondering whether we want to include this in the documentation. I've checked the main README file, it currently does not provide a complete list of all commands (e.g. `bash-it reload`). We might want to add that at one point - probably as a separate PR when (hopefully at one point) rework the documentation.

_Originally posted by @nwinkler in https://github.com/Bash-it/bash-it/pull/1672#issuecomment-709528853_

Start with the main README and review how commands are currently documented, using `bash-it reload` as an example of a missing entry. Done means the documentation provides a complete list of Bash-It commands; the issue does not identify additional files or tests.

Written by the indexing model from the issue text.