docs: use :doc: role when referencing entire pages

No need to define "cmd-foo" anchors; use :doc:`foo <cmds/foo>`
instead. If we want "cmd-foo" but it should be tested.

See also 38b24c2325 (docs: Use :doc: role when linking to commands,
2022-09-23).

doc_src/cmds/realpath.rst

@@ -1,6 +1,3 @@
-.. _cmd-realpath:
-.. program::realpath
-
 realpath - convert a path to an absolute path without symlinks
 ==============================================================