docs: Use :doc: role when linking to commands

This makes it so we link to the very top of the document instead of a
special anchor we manually include.

So clicking e.g. :doc:`string <cmds/string>` will link you to
cmds/string.html instead of cmds/string.html#cmd-string.

I would love to have a way to say "this document from the root of the
document path", but that doesn't appear to work, I tried
`/cmds/string`.

So we'll just have to use cmds/string in normal documents and plain
`string` from other commands.

doc_src/cmds/printf.rst

@@ -23,7 +23,7 @@ Description
 
 The *FORMAT* argument is re-used as many times as necessary to convert all of the given arguments. So ``printf %s\n flounder catfish clownfish shark`` will print four lines.
 
-Unlike :ref:`echo <cmd-echo>`, ``printf`` does not append a new line unless it is specified as part of the string.
+Unlike :doc:`echo <echo>`, ``printf`` does not append a new line unless it is specified as part of the string.
 
 It doesn't support any options, so there is no need for a ``--`` separator, which makes it easier to use for arbitrary input than ``echo``. [#]_ 
 
@@ -99,7 +99,7 @@ Will print "Number of bananas in my pocket: 42", `without` a newline.
 See Also
 --------
 
-- the :ref:`echo <cmd-echo>` command, for simpler output
+- the :doc:`echo <echo>` command, for simpler output
 
 Footnotes
 ---------