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/help.rst

@@ -27,7 +27,7 @@ This variable may be set as a list, where the first element is the browser comma
 Example
 -------
 
-``help fg`` shows the documentation for the :ref:`fg <cmd-fg>` builtin.
+``help fg`` shows the documentation for the :doc:`fg <fg>` builtin.
 
 Notes
 -----