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/funced.rst

@@ -48,4 +48,4 @@ When you're done, use::
 
   >_ funcsave fish_prompt
 
-For more, see :ref:`funcsave <cmd-funcsave>`.
+For more, see :doc:`funcsave <funcsave>`.