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/break.rst

@@ -17,7 +17,7 @@ Synopsis
 Description
 -----------
 
-``break`` halts a currently running loop (*LOOP_CONSTRUCT*), such as a :ref:`for <cmd-for>` or :ref:`while <cmd-while>` loop. It is usually added inside of a conditional block such as an :ref:`if <cmd-if>` block.
+``break`` halts a currently running loop (*LOOP_CONSTRUCT*), such as a :doc:`for <for>` or :doc:`while <while>` loop. It is usually added inside of a conditional block such as an :doc:`if <if>` block.
 
 There are no parameters for ``break``.
 
@@ -37,4 +37,4 @@ The following code searches all .c files for "smurf", and halts at the first occ
 See Also
 --------
 
-- the :ref:`continue <cmd-continue>` command, to skip the remainder of the current iteration of the current inner loop
+- the :doc:`continue <continue>` command, to skip the remainder of the current iteration of the current inner loop