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/cd.rst

@@ -22,7 +22,7 @@ It is recommended to keep **.** as the first element of :envvar:`CDPATH`, or :en
 Fish will also try to change directory if given a command that looks like a directory (starting with **.**, **/** or **~**, or ending with **/**), without explicitly requiring **cd**.
 
 Fish also ships a wrapper function around the builtin **cd** that understands ``cd -`` as changing to the previous directory.
-See also :ref:`prevd <cmd-prevd>`.
+See also :doc:`prevd <prevd>`.
 This wrapper function maintains a history of the 25 most recently visited directories in the ``$dirprev`` and ``$dirnext`` global variables.
 If you make those universal variables your **cd** history is shared among all fish instances.