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/end.rst

@@ -25,10 +25,10 @@ Description
 
 The **end** keyword ends a block of commands started by one of the following commands:
 
-- :ref:`begin <cmd-begin>` to start a block of commands
-- :ref:`function <cmd-function>` to define a function
-- :ref:`if <cmd-if>`, :ref:`switch <cmd-switch>` to conditionally execute commands
-- :ref:`while <cmd-while>`, :ref:`for <cmd-for>` to perform commands multiple times
+- :doc:`begin <begin>` to start a block of commands
+- :doc:`function <function>` to define a function
+- :doc:`if <if>`, :doc:`switch <switch>` to conditionally execute commands
+- :doc:`while <while>`, :doc:`for <for>` to perform commands multiple times
 
 The **end** keyword does not change the current exit status.
 Instead, the status after it will be the status returned by the most recent command.