Docs: Switch back to vanilla :ref: for commands that should be linked

Unfortunately, currently :program: doesn't link to the program's page.

So we use the old-school :ref: again where we should link, i.e. for
everything that's not the program the current page is about.

Fixes #8438

doc_src/cmds/fish.rst

@@ -1,3 +1,4 @@
+.. _cmd-fish:
 .. program::fish
 
 fish - the friendly interactive shell