move each command description into a separate document

After some other changes to ``docs/`` were merged, I offer this change to split cli.rst into a lot of command specific documents.

The motivation is very simple: when a command is changed, its documentation must follow.  If several people are working on modifying different commands, merging all those changes might become difficult.

A related benefit, each command has own page that can be easily referred to, and it's easier to read as the user does not have to "look for" a specific bit.